Patent Publication Number: US-6990652-B1

Title: System and method for determining methods and properties to be invoked on objects in a graphical program

Description:
CONTINUATION DATA 
   This is a divisional of application Ser. No. 08/810,079 titled “System and Method for Developing automation Clients Using a Graphical Data Flow Program” and filed Mar. 4, 1997, whose inventors were Murali Parthasarathy and Omid Sojoodi. 

   RESERVATION OF COPYRIGHT 
   A portion of the disclosure of this patent document contains material to which a claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but reserves all other rights whatsoever. 
   FIELD OF THE INVENTION 
   The present invention relates to graphical programming, and more particularly to accessing automation server objects by an automation client in a graphical data flow program. 
   DESCRIPTION OF THE RELATED ART 
   Traditionally, high level text-based programming languages have typically been used by programmers in writing applications programs. Many different high level programming languages exist, including BASIC, C, FORTRAN, Pascal, COBOL, ADA, APL, etc. Programs written in these high level languages are translated to the machine language level by translators known as compilers. The high level programming languages in this level, as well as the assembly language level, are referred to in this disclosure as text-based programming environments. 
   Increasingly computers are required to be used and programmed by those who are not highly trained in computer programming techniques. When traditional text-based programming environments are used, the user&#39;s programming skills and ability to interact with the computer system often become a limiting factor in the achievement of optimal utilization of the computer system. 
   There are numerous subtle complexities which a user must master before he can efficiently program a computer system in a text-based environment. For example, text-based programming environments have traditionally used a number of programs to accomplish a given task. Each program in turn often comprises one or more subroutines. Software systems typically coordinate activity between multiple programs, and each program typically coordinates activity between multiple subroutines. However, in a text-based environment, techniques for coordinating multiple programs generally differ from techniques for coordinating multiple subroutines. Furthermore, since programs ordinarily can stand alone while subroutines usually cannot in a text-based environment, techniques for linking programs to a software system generally differ from techniques for linking subroutines to a program. Complexities such as these often make it difficult for a user who is not a specialist in computer programming to efficiently program a computer system in a text-based environment. 
   The task of programming a computer system to model a process often is further complicated by the fact that a sequence of mathematical formulas, mathematical steps or other procedures customarily used to conceptually model a process often does not closely correspond to the traditional text-based programming techniques used to program a computer system to model such a process. For example, a computer programmer typically develops a conceptual model for a physical system which can be partitioned into functional blocks, each of which corresponds to actual systems or subsystems. Computer systems, however, ordinarily do not actually compute in accordance with such conceptualized functional blocks. Instead, they often utilize calls to various subroutines and the retrieval of data from different memory storage locations to implement a procedure which could be conceptualized by a user in terms of a functional block. In other words, the requirement that a user program in a text-based programming environment places a level of abstraction between the user&#39;s conceptualization of the solution and the implementation of a method that accomplishes this solution in a computer program. Thus, a user often must substantially master different skills in order to both conceptually model a system and then to program a computer to model that system. Since a user often is not fully proficient in techniques for programming a computer system in a text-based environment to implement his model, the efficiency with which the computer system can be utilized to perform such modeling often is reduced. 
   One particular field in which computer systems are employed to model physical systems is the field of instrumentation. An instrument is a device which collects information from an environment and displays this information to a user. Examples of various types of instruments include oscilloscopes, digital multimeters, pressure sensors, etc. Types of information which might be collected by respective instruments include: voltage, resistance, distance, velocity, pressure, frequency of oscillation, humidity or temperature, among others. An instrumentation system ordinarily controls its constituent instruments from which it acquires data which it analyzes, stores and presents to a user of the system. Computer control of instrumentation has become increasingly desirable in view of the increasing complexity and variety of instruments available for use. 
   In the past, many instrumentation systems comprised individual instruments physically interconnected. Each instrument typically included a physical front panel with its own peculiar combination of indicators, knobs, or switches. A user generally had to understand and manipulate individual controls for each instrument and record readings from an array of indicators. Acquisition and analysis of data in such instrumentation systems was tedious and error prone. An incremental improvement in the manner in which a user interfaced with various instruments was made with the introduction of centralized control panels. In these improved systems, individual instruments were wired to a control panel, and the individual knobs, indicators or switches of each front panel were either preset or were selected to be presented on a common front panel. 
   A significant advance occurred with the introduction of computers to provide more flexible means for interfacing instruments with a user. In such computerized instrumentation systems the user interacted with a software program executing on the computer system through the video monitor rather than through a manually operated front panel. These earlier improved instrumentation systems provided significant performance efficiencies over earlier systems for linking and controlling test instruments. 
   However, these improved instrumentation systems had significant drawbacks. For example, due to the wide variety of possible testing situations and environments, and also the wide array of instruments available, it was often necessary for a user to develop a program to control the new instrumentation system desired. As discussed above, computer programs used to control such improved instrumentation systems had to be written in conventional text-based programming languages such as, for example, assembly language, C, FORTRAN, BASIC, or Pascal. Traditional users of instrumentation systems, however, often were not highly trained in programming techniques and, in addition, traditional text-based programming languages were not sufficiently intuitive to allow users to use these languages without training. Therefore, implementation of such systems frequently required the involvement of a programmer to write software for control and analysis of instrumentation data. Thus, development and maintenance of the software elements in these instrumentation systems often proved to be difficult. 
   U.S. Pat. No. 4,901,221 to Kodosky et al discloses a graphical system and method for modeling a process, i.e. a graphical programming environment which enables a user to easily and intuitively model a process. The graphical programming environment disclosed in Kodosky et al can be considered the highest and most intuitive way in which to interact with a computer. A graphically based programming environment can be represented at level above text-based high level programming languages such as C, Pascal, etc. The method disclosed in Kodosky et al allows a user to construct a diagram using a block diagram editor such that the diagram created graphically displays a procedure or method for accomplishing a certain result, such as manipulating one or more input variables to produce one or more output variables. As the user constructs the data flow diagram using the block diagram editor, machine language instructions are automatically constructed which characterize an execution procedure which corresponds to the displayed procedure. Therefore, a user can create a text-based computer program solely by using a graphically based programming environment. This graphically based programming environment may be used for creating virtual instrumentation systems and modeling processes as well as for any type of general programming. 
   Therefore, Kodosky et al teaches a graphical programming environment wherein a user manipulates icons in a block diagram using a block diagram editor to create a data flow “program.” A graphical program for controlling instruments or implementing instrumentation functions is referred to as a virtual instrument (VI). In creating a virtual instrument, a user first creates a front panel including various controls or indicators that represent the respective input and output that will be used by the VI. When the controls and indicators are created in the front panel, corresponding icons or terminals are automatically created in the block diagram by the block diagram editor. The user then chooses various functions that accomplish his desired result, connecting the corresponding function icons between the terminals of the respective controls and indicators. In other words, the user creates a data flow program, referred to as a block diagram, representing the graphical data flow which accomplishes his desired function. This is done by wiring up the various function icons between the control icons and indicator icons. The manipulation and organization of icons in turn produces machine language that accomplishes the desired method or process as shown in the block diagram. 
   A user inputs data to a virtual instrument using front panel controls. This input data propagates through the data flow block diagram or graphical program and appears as changes on the output indicators. The data that flows from the controls to the indicators in this manner is referred to as control data. In an instrumentation application, the front panel can be analogized to the front panel of an instrument. The user adjusts the controls on the front panel to affect the input and views the output on the respective indicators. 
   In parallel to the development of graphical data flow programming for various applications, such as virtual instrumentation, object automation technology has emerged in the area of software development. The notion of object automation relates to “automation servers” “exporting” their “objects” for use by “automation clients.” An automation server is a software application which makes available its classes to be accessed from another application, referred to as an automation client. An automation client instantiates objects from the classes in the type libraries. Objects have properties and methods. Properties are the data associated with the object and typically determine attributes of the object, such as the number of columns of a spreadsheet. Methods are functions or operations which the object is capable of performing, such as drawing a spreadsheet on a display screen. An automation server exports an object by making the methods and properties of the object invokeable by an automation client. 
   An example of an automation server is Microsoft Excel®, which exports its objects for use by automation clients. An example of an automation technology is the OLE Automation interface, also known as Active X interface, IDispatch, developed by Microsoft. For example, the Microsoft Excel spreadsheet program, the Microsoft Access® database program, and the Microsoft Word® word processing program, all export objects using the OLE Automation interface. OLE Automation is an industry standard automation interface used by application programs to provide objects in a consistent manner to other application programs, development tools, and macro languages. Other examples of automation technologies are OpenDoc® and the Common Object Request Broker Architecture (CORBA). 
   Often it is desirable to display, manipulate, catalog, edit or perform other operations, such as may be performed by an automation server, on data acquired or generated by a graphical program or virtual instrument. For example, it may be desirable for a virtual instrument to display acquired temperature samples in a spreadsheet, such as a Microsoft Excel spreadsheet. More generally, it would be desirable to provide a system and method which enables a graphical data flow programming system to be able to invoke objects in automation servers for a variety of applications. 
   Therefore, improved methods are desired for enabling a graphical data flow programming system to access automation servers. More particularly, an improved system and method is desired which enables a graphical data flow program to be constructed which uses automation server objects. 
   SUMMARY OF THE INVENTION 
   The present invention provides a system and method for creating an automation client, using a graphical programming environment, wherein the automation client is operable to invoke methods and/or properties of an object instantiated from an automation class exported by an automation server. In one embodiment, the method provides a set of automation nodes and controls which a user employs to create a graphical automation client program. The automation client executes in a computer which includes a display screen and a user input device. In one embodiment, the automation client performs instrumentation control functions. 
   In one embodiment the method includes displaying on the computer screen an automation refnum, i.e., a reference to an automation class, an automation open node, and an automation invoke node and/or an automation property node in response to user input, such as the user dragging the nodes from a palette and placing or dropping the nodes in the graphical program, such as a virtual instrument block diagram. The automation refnum is used to specify the automation class. The automation open node is operable to instantiate an object of the automation class referenced by the automation refnum. The automation invoke node and automation property node are operable to invoke methods and properties, respectively, of the instantiated object. 
   The method further includes the automation refnum displaying a list of automation application type libraries of automation clients from which a user selects a type library to associate with the automation refnum. The automation refnum further displays a list of automation classes in the selected type library from which a user selects an automation class to associate with the automation refnum. The automation refnum is then connected to the automation open node in order for the refnum to provide information specifying the automation class and type library selected by the user to the open node. The automation open node uses the information to instantiate the object from the selected automation class. 
   The method further includes connecting the automation open node to the automation invoke node and/or automation property node in order for the open node to provide to the invoke node and/or property node information specifying the selected automation class and type library at edit-time and to provide a reference to the object instantiated by the open node at run-time. 
   Where an automation invoke node is used in the graphical program, the method further includes displaying a list of methods of the selected automation class from which a user selects one of the methods in the list. The automation invoke node is operable to invoke the selected method on the instantiated object at run-time. 
   Where an automation property node is used in the graphical program, the method further includes displaying a list of properties of the selected automation class from which a user selects one or more properties from the list. The automation property node is operable to invoke the selected property on the instantiated object at run-time. 
   The method further includes constructing executable instructions in response to the graphical program including the automation nodes. The executable instructions are operable to invoke methods and/or properties of the instantiated object. The method then executes the executable instructions on the computer. 
   In one embodiment, the automation technology is the OLE Automation interface. In one embodiment, the method includes displaying a list of automation technologies, such as OLE Automation, CORBA, and OpenDoc, from which a user selects an automation technology to associate with the automation refnum. 
   Thus the present invention provides a system and method for creating a graphical automation client program, using a graphical programming environment, for invoking methods and/or properties of an object instantiated from an automation class exported by an automation server. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which: 
       FIG. 1  illustrates an instrumentation control system according to the present invention; 
       FIG. 2  is a block diagram of the computer of the control system of  FIG. 1 ; 
       FIG. 3  is a block diagram illustrating the relationship of portions of the instrumentation control system of  FIG. 1  according to a first embodiment; 
       FIG. 3   a  is a block diagram illustrating the relationship of portions of the instrumentation control system of  FIG. 1  according to a second embodiment; 
       FIG. 4  is a screen shot illustrating the VI block diagram and front panel of an exemplary automation virtual instrument of  FIG. 3 ; 
       FIG. 5  is a screen shot illustrating the automation nodes palette; 
       FIG. 6  is a flowchart illustrating steps taken to create and use a graphical automation client program of  FIG. 3 ; 
       FIGS. 6   a,    6   b,  and  6   c  are a flowchart illustrating steps taken to create and use a graphical automation client program in more detail than the flowchart of  FIG. 6 ; 
       FIG. 7  is a screen shot illustrating the automation control refnum palette; 
       FIG. 8  is a screen shot illustrating an automation refnum in a VI block diagram and a pop-up menu of the automation refnum with a menu selection item for selecting an automation class; 
       FIG. 9  is a screen shot illustrating an exemplary list of the type libraries associated with the OLE Automation servers present in a system; 
       FIG. 10  is a screen shot illustrating a type library browse menu selected from the pop-up menu of  FIG. 8 ; 
       FIG. 11  is a screen illustrating an automation class having been chosen for the automation refnum of FIG.  8  and an automation open node being wired to the automation refnum; 
       FIG. 12  is a screen shot illustrating an automation property node wired to the automation open node of  FIG. 11 and a  list of properties of the automation class which was chosen in  FIG. 1 ; 
       FIG. 13  is a screen shot illustrating a property having been chosen for the automation property node of  FIG. 12 ; 
       FIG. 14  is a screen shot illustrating an automation invoke node wired to the automation property node of  FIG. 12 and a  list of methods of the automation class which was chosen in  FIG. 12 ; 
       FIG. 15  is a screen shot illustrating a method having been chosen for the automaton invoke node of  FIG. 14 ; 
       FIG. 16  is a flowchart illustrating steps taken to display an application help screen for an automation object according to the preferred embodiment of the present invention; 
       FIG. 17  is a screen shot illustrating the step of displaying a method of an automation invoke node according of the flowchart of  FIG. 16 ; 
       FIG. 18  is a screen shot illustrating the step of displaying an application help option for a method of an automation invoke node according of the flowchart of  FIG. 16 ; 
       FIG. 19  is a screen shot illustrating the step of displaying a application help screen of the flowchart of  FIG. 16 ; 
       FIG. 20  is a flowchart illustrating steps taken to propagate the automation class of an automation node to another automation node; 
       FIGS. 21 through 23  are screen shots illustrating steps of the flowchart of  FIG. 20 ; 
       FIG. 24  is a flowchart illustrating steps taken to propagate the automation class of an automation node to another automation node; 
       FIGS. 25 and 26  are screen shots illustrating steps of the flowchart of  FIG. 24 ; 
       FIG. 27  is a flowchart illustrating steps taken to perform type propagation checking of automation nodes; 
       FIGS. 28 through 30  are screen shots illustrating steps of the flowchart of  FIG. 27 ; 
       FIG. 31  is a flowchart illustrating in more detail the step of  FIG. 27  of performing type propagation; 
       FIGS. 32 ,  33  and  34  are screen shots illustrating a help screen for an automation open node, an automation property node, and an automation invoke node, respectively. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Incorporation By Reference 
   U.S. patent application Ser. No. 08/716,725 titled “System and Method for Performing Interface Independent Virtual Instrumentation Functions in a Graphical Data Flow Program” filed Sep. 23, 1996, whose inventors were Omid Sojoodi and Scott A. Rust, and which is assigned to National Instruments Corporation, is hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
   U.S. patent application Ser. No. 08/717,771 titled “System and Method for Performing Class Checking of Objects in a Graphical Data Flow Program” filed Sep. 23, 1996, whose inventors were Omid Sojoodi and Steven W. Rogers, and which is assigned to National Instruments Corporation, is hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
   U.S. patent application Ser. No. 08/717,772 titled “System and Method for Performing Instrumentation Independent Virtual Instrumentation Functions Using Attribute Nodes in a Graphical Data Flow Program” filed Sep. 23, 1996, whose inventors were Omid Sojoodi and Steven W. Rogers, and which is assigned to National Instruments Corporation, is hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
   U.S. Pat. No. 5,481,741 titled “Method and Apparatus for Providing Attribute Nodes in a Graphical Dataflow Environment” is hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
   Instrumentation Control System 
   Referring now to  FIG. 1 , an instrumentation system  10  is shown. The present invention may be used in any of various types of graphical programming systems and graphical data flow programming systems. For example, the present invention may be used in a general purpose graphical programming system or general purpose graphical data flow programming system. In the preferred embodiment, the invention is used in a graphical programming system for virtual instrumentation, and this embodiment is described below. 
   The system  10  comprises a computer  12 , which connects to one or more instruments. The one or more instruments may include a GPIB (general purpose interface bus) instrument  14 , a VXI (VME eXtension for Instrumentation) chassis  16  comprising one or more VXI card instruments, a serial instrument  18  and/or a data acquisition board  20 . The GPIB instrument  14  is coupled to the computer  12  via a GPIB interface provided by the computer  12 . The VXI instrument  16  is coupled to the computer  12  via a VXI bus or MXI bus provided by the computer. The serial instrument  18  is coupled to the computer  12  through a serial port, such as an RS-232 port, provided by the computer  12 . Finally, the data acquisition board  20  is coupled to the computer  12 , typically by being plugged in to an I/O slot in the computer such as a PCI bus slot, an ISA bus slot, an EISA bus slot, or a MicroChannel bus slot provided by the computer  12 . In typical instrumentation control systems an instrument will not be present of each interface type and in fact many systems may only have one or more instruments of a single interface type, such as only GPIB instruments. The term “instrument” used herein also includes software code or software objects which implement instrument functionality or are used to control instruments. 
   The instruments are coupled to a unit under test (UUT)  23 , a process, or are coupled to receive field signals, typically generated by transducers. The system  10  may be used in a data acquisition and control application, or may instead be used in a test and measurement application. If the system  10  is used in a data acquisition application, the system  10  also preferably includes signal conditioning circuitry  21  coupled between the data acquisition board  20  and transducers. 
   The system  10  preferably includes a memory media, such as magnetic media, or floppy disks  22 , on which computer programs according to the present invention are stored. In one embodiment of the system  10 , software programs according to the present invention are also stored on a memory or hard drive of the computer and executed by a CPU of the computer. The CPU executing code and data from the memory thus comprises a means for performing various instrumentation control functions according to the steps described below. In the present disclosure, the term “computer-readable storage media” is intended to include any of various types of memory media or machine readable media for storing data, including floppy disks, CDs or CD-ROMs, and other distributable media, as well as computer memories such as hard drives, optical storage, DRAM (dynamic RAM) or other types of computer memory. 
   Computer Block Diagram 
   Referring now to  FIG. 2 , a block diagram of the computer  12  (of  FIG. 1 ) is shown. The elements of a computer not necessary to understand the operation of the system  10  have been omitted for simplicity. The computer  12  includes at least one central processing unit or CPU  24  which is coupled to a processor or host bus  26 . The CPU  24  may be any of various types, including an x86 processor, a PowerPC processor, a CPU from the Motorola family of processors, a CPU from the SPARC family of RISC processors, as well as others. Main memory  28  is coupled to the host bus  26  by means of memory controller  30 . The main memory  28  stores the various portions of instrumentation control software, including, inter alia, instrumentation control application software, automation servers, automation clients, code and data for automation nodes and controls, an object manager for managing objects, and object type libraries including class information about the classes exported by the automaton servers. The main memory  28  also stores operating system software as well as the software for operation of the computer system, as well known to those skilled in the art. The instrumentation control software will be discussed in more detail below. 
   The host bus  26  is coupled to an expansion or input/output bus  32  by means of a bus controller  34 . The expansion bus  32  includes slots for various devices such as the data acquisition board  20  (of FIG.  1 ), a GPIB interface card  36  which provides a GPIB bus interface  38  to the GPIB instrument  14  (of FIG.  1 ), and an MXI bus card  40  coupled to a VXI chassis  16  for receiving VXI card instruments (of FIG.  1 ). The computer  12  further comprises a video controller  44  and hard drive  46  coupled to the expansion bus  32 . A video display  45  is coupled to the video controller  44  for displaying video information. The computer  12  further comprises a mouse  49  and keyboard  47  for receiving user input. 
   Graphical Programming Environment 
   Referring now to  FIG. 3 , a block diagram illustrating the relationship of portions of the instrumentation control system  10  (of  FIG. 1 ) are shown. Preferably, the elements shown in  FIG. 3  (with the exception of the hardware instrument  54 ) are software elements which are executed on the computer  12  (of FIG.  1 ). The present invention is used to create an automation graphical program. The present invention is also useable to create an automation graphical program portion which is a part of a larger graphical program. 
   In the preferred embodiment, a programmer employs a front panel editor  62 , a block diagram editor  64 , and a connector pane/icon editor  72  of a graphical programming environment to produce an automation graphical program. In the instrumentation application of the preferred embodiment, the graphical program is referred to as an automation virtual instrument (VI)  50 . The block diagram editor  64  generates executable instructions, i.e., machine language instructions, in response to the VI  50 . The VI  50  developed by the programmer is executed by an execution subsystem  56  of the graphical programming environment to control an instrument  54 . The instrument  54  is illustrative of instruments such as those of FIG.  1 . 
   Referring now to  FIG. 4 , a screen shot from a graphical programming environment including an automation virtual instrument or graphical program exemplary of the VI  50  of  FIG. 3  is shown. The screen shot of  FIG. 4  comprises an instrument front panel in a window in the upper portion of the screen and a block diagram in a window in the lower portion of the screen. The block diagram comprises program execution elements, referred to as nodes, which are wired or linked together to produce a data flow program. The front panel comprises controls for providing input data to the block diagram and indicators for receiving and/or displaying output data from the nodes of the block diagram. 
   Preferably the graphical programming system comprises portions of National Instruments LabVIEW software. The drawings of the present disclosure include numerous screen shots displayed during the execution of LabVIEW. In the screen shots, LabVIEW is executing under the supervision of the MICROSOFT WINDOWS 95 operating system. For more information on the LabVIEW graphical programming environment of the preferred embodiment, please see U.S. Pat. No. 5,481,741 referenced above. 
   Referring again to  FIG. 3 , the graphical programming environment further comprises an automation control  74 . The front panel editor  62  is operable to generate to a VI front panel. The front panel editor  62  communicates with the automation control  74 . The automation control  74  communicates with an object manager  68  to obtain or provide information regarding automation type libraries  70  in the system and automation classes in the automation type libraries  70 . 
   The graphical programming environment further comprises automation nodes  66 . The automation nodes  66  are shown in the automation nodes palette of FIG.  5 . The automation nodes  66  comprise an automation refnum, an automation open node, an automation close node, an automation invoke node and an automation property node. The block diagram editor  64  is operable to create a VI block diagram, or block diagram. The block diagram editor  64  communicates with the automation nodes  66 , which in turn communicate with the object manager  68 . 
   The object manager  68  accesses automation type libraries  70  to acquire information necessary to perform object management operations. Preferably, the object manager  68  communicates with the Windows operating system Registry to obtain information regarding the automation type libraries  70  in the system. The automation control  74 , automation nodes  66 , and object manager  68  will be discussed in more detail below. 
   Advantageously, the graphical programming environment, and in particular the automation control  74 , the automation nodes  66 , the object manager  68 , and the automation manager run-time library  58 , enable a graphical program to instantiate objects of an unlimited number of automation classes of automation applications, and to remotely invoke properties and methods of the instantiated automation objects. 
   The graphical programming environment further preferably comprises a connector pane/icon editor  72  for forming VI&#39;s into subVI&#39;s, i.e., a VI which may be used as a graphical programming element in another VI. The reader is referred to U.S. Pat. No. 5,301,336 for more information about the subVI&#39;s and the icon editor  72 . 
   The execution subsystem  56  executes the executable instructions constructed from a block diagram of the VI  50 . For more information about the execution subsystem  56  the reader is referred to U.S. Pat. No. 5,481,741. 
   Preferably, the VI  50  invokes methods and properties of objects of an automation server  52 , or more than one automation server, indirectly through the services of an automation manager run-time library  58 . Examples of the automation server  52  include Microsoft Excel, Access, Word, and National Instruments ComponentWorks controls. Other examples of the automation server  52  include Claris Works and Cyberdog. The VI  50  controls the instrument  54  through an instrument device driver  53  which includes executable functions which are called by the VI  50  to perform operations in order to control the instrument  54 . 
   The automation nodes  66  and automation control  74  comprise classes and objects, according to the notion of classes and objects in the art of object-oriented programming. Each of the automation nodes  66  and the automation control  74  comprise properties and methods. The methods include a method for drawing an icon representation of the object on the video display  45  of the computer  12  either in the VI block diagram or front panel, a method for generating code associated with the different functions of each node or control, and a method for performing type propagation checking. The operation of automation nodes  66  and automation control  74  will be explained in more detail below. As mentioned above, the automation nodes  66  comprise an automation refnum associated with the automation control  74 , an automation open node, an automation property node, an automation invoke node, and an automation close node. 
     FIG. 3   a  illustrates an alternate embodiment of the system  10  of FIG.  3 . The system  10  is similar to that shown in FIG.  1  and corresponding elements are numbered identically. In the embodiment of  FIG. 3   a  the automation server  52  is a program capable of controlling an instrument  54 . For example, the automation server  52  may be a program written in the C language for controlling a GPIB instrument. The VI  50  instantiates objects from the classes exported by the automation server  52  and invokes methods and properties of the objects. 
   As mentioned above, the graphical program  50  is not necessarily related to controlling an instrument, but rather the graphical program  50  may be for any of various applications. That is, the automation client may have an application other than instrumentation control. In one embodiment similar to that of  FIG. 3   a,  the VI  50  is an automation client application which performs other functions. For example, a programmer desires to develop a stock portfolio viewer in a graphical programming environment and develops an automation client to access Microsoft Excel spreadsheet objects in order to display a stock portfolio. 
   Advantageously, the graphical system and method for producing the graphical program or VI  50  has a number of benefits. These benefits include reduction in the development time required to create the VI  50  as well as reduction of the number of code defects in the VI  50 . Yet another benefit is the simplicity of programming which makes the development of a graphical program, such as an instrumentation control program, more practical for a larger number of people, i.e., those who might not have the skills, or resources to develop the skills, to develop programs according to more conventional text-based methods. The system and method also provides class propagation, class checking and type checking in a graphical programming environment, discussed further below, thus simplifying program development. 
   Automation Client Creation 
   Referring now to  FIG. 6 , a flowchart illustrating steps taken to create a graphical automation client according to the preferred embodiment of the method is shown. The flowchart of  FIG. 6  provides a high level view of the method of the preferred embodiment. The flowchart of  FIGS. 6   a - 6   c  provide a more detailed view of the method of the present invention. Thus the flowchart of  FIG. 6  provides a high level view of the steps of the flowchart of  FIGS. 6   a,    6   b,  and  6   c,  and corresponding steps are numbered identically. The user drops or places an automation control icon on a virtual instrument front panel, and in response the block diagram editor  64  displays a corresponding automation class refnum icon in the block diagram, in step  80 . The user then selects an automation class to associate with the automation refnum. The automation refnum receives the user input and associates the user-selected automation class with the automation refnum, in step  92 . 
   The user drops an automation open node on the block diagram and in response the block diagram editor  64  displays an automation open node icon, in step  94 . The user connects the automation refnum and the automation open node and in response the block diagram editor  64  displays a wire connecting the automation refnum and the automation open node, in step  96 . 
   The user drops an automation function node in the block diagram and in response the block diagram editor  64  displays an automation function node, in step  98 . An automation function node preferably comprises an automation property node or an automation invoke node. In one embodiment, an automation function node may perform functions of automation objects other than invoking methods and properties, such as event handling, i.e., the automation function node may be an automation event node. The user connects the automation open node and the automation function node and in response the block diagram editor  64  displays a wire connecting the automation open node and the automation function node, in step  100 . The user may drop both an automation invoke node and an automation property node in creating the graphical automation client. Furthermore, the user may drop one or more of both automation property nodes and automation invoke nodes in order to invoke multiple properties and/or methods in creating the graphical automation client. 
   The user then selects a property or method and in response the automation property node or automation invoke node receives the user input and associates the user-selected property or method, respectively, with the automation property node or automation invoke node, respectively, in step  106 . In the case of the automation property node, the user may select multiple properties to associate with the automation property node. 
   The graphical programming environment then constructs execution instructions based on the graphical program comprising the automation refnum, automation open node, automation property node and/or automation invoke node, and wires, in step  126 . The graphical programming environment then executes the execution instructions, in step  128 . A more detailed description of the steps of  FIG. 6  is given below with regard to  FIGS. 6   a,    6   b,  and  6   c.    
   Referring now to  FIGS. 6   a,    6   b,  and  6   c,  a flowchart illustrating steps taken to create a graphical automation client according to the preferred embodiment of the method in more detail than  FIG. 6  is shown. A user drags, preferably using a mouse, an automation control from a refnum palette, shown in  FIG. 7 , and drops the automation control on a VI front panel. 
   In response to the user dropping the automation refnum, the front panel editor  62  displays an automation control in the front panel, and the block diagram editor  64  displays an automation refnum associated with the automation control in a block diagram in step  80 . Alternatively, the user drops an automation refnum from the palette of FIG.  5  and the front panel editor  62  invokes a method of the automation control  74  to draw an automation control icon in the front panel. 
   Preferably, the automation control  74  comprises a draw method which the front panel editor  62  and block diagram editor  64  invoke in order to display an automation control icon in the front panel and to display an automation refnum icon in the block diagram, respectively.  FIG. 8  shows an automation refnum displayed in a block diagram.  FIG. 8  also shows a pop-up menu for the automation refnum including a “Select OLE Class” menu item with a “Browse” item. Preferably, the user right clicks a mouse on the automation refnum in order to see the pop-up menu. 
   In one embodiment, an automation client program, i.e., a VI, developed using the graphical programming environment is capable of invoking methods of and modifies attributes of objects via a plurality of automation technologies, including Microsoft Automation. Examples of other automation technologies are OpenDoc and the Common Object Request Broker Architecture (CORBA). In this embodiment, the object manager  68  obtains a list of automation technologies in the system. The automation refnum queries the object manager  68  for the list and displays the list of automation technologies for the user in step  82 . The user selects one of the automation technologies from the displayed list of automation technologies. In response to the user selection, the automation refnum selects the automation technology from the list of automation technologies and associates the selected automation technology with the automation refnum in step  84 . 
   The automation refnum is a reference to a user-selected automation class. The automation refnum includes a refnum output terminal, to which a wire may be connected. At edit-time, the automation refnum provides, at its output terminal, a type descriptor which specifies an automation class and the type library to which the automation class belongs. The type library is one of the type libraries  70  of FIG.  3 . Type descriptors will be described in detail below. The time during which a user is creating, i.e., editing a VI  50 , by dropping nodes and wiring them together, is referred to as “edit-time.” The time when instructions of the VI  50  are executed is referred to as “run-time”. 
   Referring again to  FIG. 6 , in response to user input, the automation refnum queries the object manager  68  for a list of type libraries  70  (of  FIG. 3 ) associated with the automation servers present in the system. The automation refnum displays the list of type libraries  70  associated with the automation servers in step  86 . Preferably, the object manager  68  provides the automation refnum with a list of OLE Automation type libraries in the system. In one embodiment, the object manager  68  provides the automation refnum with a list of type libraries for each automation technology in the system. 
   Preferably, the user input includes the user clicking on the “Browse” item of the “Select OLE class” item of the automation refnum pop-up menu, shown in  FIG. 8 , and selecting the pull-down menu in the “Type Library” window shown in FIG.  10 . Preferably, the object manager  68  queries the Windows Registry to obtain a list of OLE Automation type libraries present in the system. Preferably, the automation refnum displays the type libraries associated with the OLE Automation servers present in the system, as shown in FIG.  9 . In one embodiment, the automation refnum displays the type libraries associated with the automation technology which was selected in step  84 . 
   The user selects one of the type libraries from the displayed list of type libraries. In response, the automation refnum selects the type library from the list of type libraries and associates the selected type library with the automation refnum in step  88 . At edit-time the automation refnum provides a type descriptor, including information identifying the selected type library, to other automation nodes of the VI  50 .  FIG. 10  shows the user having selected the “Microsoft Excel 5.0 Object Library Version 1.0” type library from the list of FIG.  9 . 
   In response to the user having selected a type library, the automation refnum queries the object manager  68  for a list of possible automation classes associated with the selected type library in step  90 . The object manager  68  queries the selected type library for a list of possible automation classes in the type library. The object manager  68  receives the list of automation classes from the type library and provides the list to the automation refnum. The automation refnum receives the list from the object manager  68  and displays the list, as shown in FIG.  10 .  FIG. 10  shows the list of possible Microsoft Excel 5.0 automation classes from which automation objects may be instantiated. Preferably, the user may choose for the automation refnum to selectively display only those objects which are creatable, as shown in FIG.  10 . 
   The user selects an automation class from the displayed list of automation classes. In response, the automation refnum selects the automation class from the list of automation classes and associates the selected automation class with the automation refnum in step  92 . At edit-time the automation refnum provides a type descriptor, including information identifying the selected automation class, to other automation nodes of the VI  50 .  FIG. 11  shows the user having selected the “Excel Application” automation class from the list of FIG.  10 . Thus, the system and method displays an automation refnum icon which is used to indicate an input type library and automation class. 
   Below an automation open node is described in detail. In one embodiment, the system and method displays the automation open node icon which is used to indicate the type library and automation class information input from a user, rather than the automation refnum. That is, the automation open node serves the function of the automation refnum by receiving the type library and automation class information from the user and displaying the selected information. 
   Type Descriptor 
   Each wire and terminal in a block diagram has an associated data type. The programming environment keeps track of the data type in a structure in memory called a type descriptor. The type descriptor comprises a string of word integers that describe the data type. The generic format of a type descriptor is:
     &lt;size&gt; &lt;typecode&gt;.   

   Table 1 lists three of the supported data types, the type codes, and type descriptors in one embodiment of the programming environment. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Data Type 
               Type Code 
               Type Descriptor 
             
             
                 
             
           
          
             
               Long Intefer 
               0x03 
               0004 xx03 
             
             
               Handle 
               0x31 
               0006 xx31 &lt;kind&gt; 
             
             
               Array 
               0x40 
               &lt;nn&gt; 0x40 &lt;k&gt; &lt;k dimensions&gt; 
             
             
                 
                 
               &lt;element type descriptor&gt; 
             
             
                 
             
          
         
       
     
   
   When a wire is initially connected to a terminal, the wire takes on the data type of the terminal. i.e., the programming environment creates a type descriptor for the wire. When the user connects this wire to another terminal in the block diagram at edit-time, the programming environment performs type propagation checking by comparing the type descriptor of the wire with the type descriptor of the terminal. If the type descriptors do not match, then a type conflict error is generated. In one embodiment, the programming environment performs type propagation checking on each wire and terminal in the block diagram each time a change is made to the diagram. That is, type descriptors are propagated on the wires of the block diagram at edit-time in order to perform type propagation checking. 
   The method advantageously comprises a new type descriptor for the automation refnum terminal. The automation refnum terminal type descriptor includes an identifier for the automation class associated with the automation refnum and an identifier for the type library for the automation class. In one embodiment, the automation refnum type descriptor has the format:
     &lt;size&gt; &lt;RefnumCode&gt; &lt;AutoRefnumKind&gt; &lt;AutomationType&gt;   &lt;no of int16&#39;s&gt; &lt;kCoClassCLSID&gt; &lt;CLSID of created object&gt;   &lt;kTypeLibCLSID&gt; &lt;CLSID of type library&gt; &lt;DISPID&gt;.
 
The &lt;size&gt; byte of the type descriptor is as described above. The &lt;refnumCode&gt; is the type code for a refnum. The &lt;AutoRefnumKind&gt; value distinguishes this refnum from other refnums as an automation refnum. The &lt;AutomationType&gt; indicates the OLE automation type, such as the &lt;kStOLEAutoType&gt; value which indicates a static OLE automation type. The &lt;no of int16&#39;s&gt; field indicates the number of 16 bit words which follow. The &lt;kCoClassCLSID&gt; value indicates the following 128 bits are a class identifier. The &lt;CLSID of created object&gt; is a unique 128 bit number associated with the particular automation class which the automation refnum references. The &lt;kTypeLibCLSID&gt; value indicates the following 128 bits are a type library identifier. The &lt;CLSID of type library&gt; is a unique 128 bit number associated with the particular type library to which the automation class belongs. The &lt;DISPID&gt; is a Dispatch ID, which is a long integer which uniquely specifies a class within a type library. The Dispatch ID is associated with the Microsoft IDispatch interface for dispatch methods and properties. The Dispatch ID is unique within a type library. In the example shown in  FIG. 11 , the type descriptor provided by the automation refnum includes information to specify the Microsoft “Excel” automation application type library and the “Application” automation class.
   

   The automation nodes comprise a type propagation checking method which may be invoked by the block diagram editor  64  to perform type propagation checking. When the user connects a wire to a terminal of an automation node, the type propagation method of the node is invoked and the type descriptor of the wire being connected to the terminal is passed as an argument to the type propagation method. This information in the type descriptor enables the type propagation method to advantageously determine class conflicts in the block diagram. 
   Thus, the method advantageously performs type propagation checking to determine program correctness when wiring automation function nodes. This checking advantageously prevents run-time errors which would occur when the user attempted to invoke a method or property which is invalid for the automation class selected. Type propagation checking is discussed further below. 
   The user creates an automation open node for instantiating an object from the automation class referred to by the automation refnum. Preferably, the user drags an automation open node from the automation nodes palette of FIG.  5  and drops the automation open node on the block diagram. 
   In response to the user dropping the automation open node on the block diagram, the block diagram editor  64  displays an automation open node in the block diagram, as shown in  FIG. 11 , in step  94 . Preferably, the block diagram editor  64  invokes a draw method of the automation open node to display an automation open node icon in the block diagram. At run-time, the automation open node instantiates an object based on the automation class and type library information received from the automation refnum. 
   The system then connects the automation refnum to the automation open node in response to user input. The user wires the automation open node to the automation refnum using a wiring tool. In response, the block diagram editor  64  displays a wire connecting the automation open node and the automation refnum in step  96 . The automation refnum provides the automation class and type library information to the automation open node so that the automation open node may perform type propagation checking. 
   The automation open node includes a refnum input terminal which is designed to receive an output from an automation refnum icon. In an embodiment where an automation refnum icon is not used to designate class and type library information, the automation open node receives the class and type library information by other means, preferably via user input in a similar manner which the automation refnum received the class and type library information. At edit-time, the automation open node receives a type descriptor on the wire, such as from the automation refnum, which includes type library and automation class information. The automation open node also includes a refnum output terminal, to which a wire may be connected. At edit-time, the automation open node forwards the type descriptor received at its refnum input terminal to the refnum output terminal. The type descriptor is forwarded on a connected wire to all other nodes connected to the wire. 
   At run-time, the automation open node provides a reference to the object instantiated by the automation open node on the wire connected to the refnum output terminal. Automation nodes, such as automation invoke nodes and automation property nodes, receive the object reference in order to invoke methods and properties of the object as will be described below. 
   The user creates an automation property node in order to invoke properties, i.e., to set or get properties, of the object instantiated by the automation open node. Preferably, the user drags an automation property node from the automation nodes palette of FIG.  5  and drops the automation property node on the block diagram, as shown in FIG.  12 . The user may also create an automation property node via the pop-up menu of the automation open node. 
   In response to the user dropping the automation property node, the block diagram editor  64  invokes a method on the automation property node to display an automation property node icon in the block diagram in step  98   a.  The automation property node is used to invoke properties of the object. The properties to be invoked by the automation property node are selected by the user creating the VI  50 . 
   The user wires the automation property node to the automation open node using a wiring tool. The automation property node includes a refnum input terminal through which the automation property node receives an object reference and type descriptor. In response to the user wiring the automation property node and the automation open node, the block diagram editor  64  displays a wire connecting the refnum input terminal of the automation property node and the refnum output terminal of either the automation open node or an automation invoke node in step  100   a.    
   It is noted that the refnum input terminal of the automation property node may instead be connected to other wires which provide the reference to the object and type descriptor rather than the automation open node refnum output terminal. For example, the refnum input terminal of the automation property node may be connected to the refnum output terminal of another automation property node of the same object or to the refnum output terminal of an automation invoke node of the same object. 
   At run-time, the automation property node receives a reference to the instantiated object via its refnum input terminal so that the automation property node may set or get properties of the instantiated object. 
   At edit-time, the automation property node receives a type descriptor via its refnum input terminal so that the automation property node may perform type propagation checking. The automation property node also uses the information in the type descriptor at edit-time to perform other operations, such as displaying property lists as described below. 
   The automation property node also includes a refnum output terminal. The automation property node passes the information received on its refnum input terminal to its refnum output terminal, i.e., the type descriptor, at edit-time and the object reference at run-time. 
   Preferably, the user selects the “Properties” menu item from the automation property node pop-up menu in order to view a list of properties associated with the automation class, as shown in FIG.  12 . In response to the user selecting the Properties item, the automation open node (or other automation node) provides information on the selected automation class and selected type library to the automation property node in step  102 . Preferably, providing information on the automation class and selected type library includes providing a type descriptor which includes the information. 
   Using the automation class and type library information, the automation property node queries the object manager  68  for a list of properties associated with the selected automation class in step  103 . In response, the object manager  68  queries the selected type library for a list of properties of the specified automation class in step  104 . The object manager  68  receives the list of properties from the type library and provides the list to the automation property node. The automation property node uses the information received from the object manager  68  to display the list of properties of the selected automation class, as shown in  FIG. 12 , in step  105 . 
   The user selects one of the properties in the displayed list of properties. In response to the user selection, the automation property node selects the selected property to be invoked by the automation property node in step  106   a.  The automation property node displays the selected property in the automation property node, as shown in  FIG. 13 , in step  108 .  FIG. 13  shows a “Visible” property displayed in the Excel Application automation property node. 
   The automation property node displays a terminal for the selected property, as shown in  FIG. 13 , in step  110 . If the property may be set, i.e., written, the automation property node displays an input terminal. If the property may be gotten, i.e., read, the automation property node displays an output terminal. Preferably, a property may be set to be readable or writable in response to user input. In  FIG. 13 , the Visible property includes an input terminal since the property may be set, i.e., changed. The Visible property input terminal receives a Boolean, i.e., True/False, input value which may be toggled on the front panel in response to user input. 
   In a similar manner, the user is enabled to add more properties to the automation property node for the purpose of invoking the properties of the object. Typically, the properties are set via controls on the front panel associated with the block diagram or gotten and displayed on indicators on the front panel. In addition, the properties may be invoked programmatically using nodes and terminals of the VI  50 . 
   The user creates an automation invoke node in order to invoke methods of the object instantiated by the automation open node. Preferably, the user drags an automation invoke node from the automation nodes palette of FIG.  5  and drops the automation invoke node on the block diagram, as shown in FIG.  14 . The user may also create an automation invoke node via the pop-up menu of the automation open node. 
   In response to the user dropping the automation invoke node, the block diagram editor  64  invokes a method on the automation invoke node to display an automation invoke node icon in the block diagram in step  98   b.  The automation invoke node is used to invoke methods of the object. The methods to be invoked by the automation invoke node are selected by the user creating the VI  50 . 
   The user wires the automation invoke node to the automation open node using a wiring tool. The automation invoke node includes a refnum input terminal through which the automation invoke node receives an object reference and type descriptor. In response to the user wiring the automation invoke node and the automation open node, the block diagram editor  64  displays a wire connecting the refnum input terminal of the automation invoke node and the refnum output terminal of either the automation open node or an automation property node in step  100   b.    
   It is noted that the refnum input terminal may instead be connected to other wires which provide the reference to the object and type descriptor rather than the automation open node refnum output terminal. For example, the refnum input terminal of the automation invoke node may be connected to the refnum output terminal of another automation invoke node of the same object or to the refnum output terminal of an automation property node of the same object. 
   At run-time, the automation invoke node receives a reference to the instantiated object via its refnum input terminal so that the automation invoke node may invoke methods of the instantiated object. 
   At edit-time, the automation invoke node receives a type descriptor via its refnum input terminal so that the automation invoke node may perform type propagation checking. The automation invoke node also uses the information in the type descriptor at edit-time to perform other operations, such as displaying method lists as described below. 
   The automation invoke node also includes a refnum output terminal. The automation invoke node passes the information received on its refnum input terminal to its refnum output terminal, i.e., the type descriptor, at edit-time and the object reference at run-time. 
   Preferably, the user selects the “Methods” menu item from the automation invoke node pop-up menu in order to view a list of invoke associated with the automation class, as shown in FIG.  14 . In response to the user selecting the Methods item, the automation open node (or other automation node) provides information on the selected automation class and selected type library to the automation invoke node in step  116 . Preferably, providing information on the automation class and selected type library includes providing a type descriptor which includes the information. 
   Using the automation class and type library information, the automation invoke node queries the object manager  68  for a list of methods associated with the selected automation class in step  117 . In response, the object manager  68  queries the selected type library for a list of methods of the specified automation class in step  118 . The object manager  68  receives the list of methods from the type library and provides the list to the automation invoke node. The automation invoke node uses the information received from the object manager  68  to display the list of methods of the selected automation class, as shown in  FIG. 14 , in step  119 . 
   The user selects one of the methods in the displayed list of methods. In response to the user selection, the automation invoke node selects the selected method to be invoked by the automation invoke node in step  106   b.  The automation invoke node displays the selected method in the automation invoke node, as shown in  FIG. 15 , in step  122 .  FIG. 15  shows a “Workbooks” method displayed in the Excel Application automation invoke node. 
   The automation invoke node displays a terminal for each of the parameters of the selected method, as shown in  FIG. 15 , in step  124 . If the method includes input parameters, the automation invoke node displays an input terminal for each input parameter. If the includes output parameters, the automation invoke node displays an output terminal for each of the output parameters. In  FIG. 15 , the Workbooks method includes an “Index” parameter. An input terminal and an output terminal are displayed for the Index parameter since it is both an input and output parameter. The Index input terminal receives an integer input value which may be specified on the front panel in response to user input. In addition, the Workbooks method returns an output parameter, and  FIG. 15  displays an output terminal for the Workbooks method. 
     FIG. 15  also shows an automation close node wired to the automation invoke node for freeing, i.e., deconstructing, the instantiated object at run-time referenced on the wire connecting the automation invoke node refnum output terminal and the automation close node refnum input terminal. 
   It is noted that the order in which an automation refnum, an automation open node, an automation property node, and an automation invoke node are dropped, wired, and configured is not fixed. For example, the automation open node could be dropped first. The automation open node includes a “Configure” item in its pop-up menu which automatically creates a configured automation control and associated automation refnum, and wires the automation refnum output terminal to the automation open node refnum input terminal. In another example, an automation property node and an automation invoke node may be wired together in “parallel”. That is, the automation refnum input terminal of each of an automation invoke node and an automation property node may be wired to the same wire emanating from the automation refnum output terminal of an automation open node. 
   Furthermore, a graphical automation client may be created with just an automation invoke node and no automation property node or a graphical automation client may be created with just an automation property node and no automation invoke node. That is, steps  98   a  through  110 , and steps  126  and  128  could be performed without steps  98   b  through  124  being performed. Likewise, steps  98   b  through  128  could be performed without steps  98   a  through  110  being performed. 
   Once the user has created the VI  50 , the user instructs the graphical programming environment to construct execution instructions in accordance with the nodes in the block diagram in step  126 . Preferably, constructing the execution instructions comprises generating machine language instructions into an executable program. Alternatively, constructing the execution instructions comprises generating programming language instructions, such as C language instructions, and compiling the programming language instructions into an executable program. In one embodiment, the user causes the execution instructions to be generated by clicking on the run button, indicated by a double wide right-pointing arrow at the far left of the toolbar of the front panel of FIG.  15 . In addition, the user causes the execution instruction to be generated by saving the virtual instrument to a file via the File pull-down menu of FIG.  15 . 
   Constructing the execution instructions includes invoking code generation methods for each of the automation nodes. The automation open node generates instructions to instantiate an object from the automation class indicated by the automation refnum. Preferably, the automation class and type library information is included in the execution instructions generated by the automation open node. The automation property node generates instructions for invoking the properties of the automation property node. The instructions set properties according to the values supplied at corresponding property input terminals and get property values and provide the values to corresponding output terminals. The automation invoke node generates instructions for invoking the selected method of the automation invoke node. The instructions invoke the method with the values supplied at the parameter input terminals and supply return values at the output terminals. 
   Once the execution instructions have been generated, the user directs the environment to execute the execution instructions. In response, the execution subsystem  56  (of  FIG. 3 ) executes the execution instructions of the VI  50  in step  128 . Preferably the user clicks the run button, or selects the run menu item from the Operate menu of  FIG. 15 , to execute the execution instructions. In one embodiment, the execution instructions are interpreted rather than compiled. 
   Thus, the method enables a user to create an automation client application by means of a graphical programming environment to instantiate objects from automation classes and invoke methods and properties of the automation objects. 
   Context-Sensitive Help Display 
   Referring now to  FIG. 16 , a flowchart illustrating steps taken to display an application help screen for an automation object according to the preferred embodiment is shown. A user creates a VI, such as the VI  50  of  FIG. 3 , according to the steps of the flowchart of FIG.  6 . The VI includes an automation invoke node and/or an automation property node, referred to generically as an automation node. The block diagram editor  64  displays a method in an automation invoke node or a property in an automation property node, as shown in  FIG. 17 , in step  140 . In particular,  FIG. 17  shows a “Workbooks” method of “Application” automation class from the type library of an “Excel” automation server, i.e., an automation application. 
   The user provides user input, preferably right-clicking with a mouse on the automation node, to view a pop-up menu. In response to the user input, the automation node displays an application help option for the method/property indicated by the user input, as shown in  FIG. 18 , in step  142 . Preferably, displaying an application help option includes displaying a pop-up menu with a help item. In  FIG. 18  the help item is the “Help for Workbooks” item. 
   The user selects the help item, preferably by clicking on the help item with a mouse. In response to the user selection of the help item, the automation node requests help screen information for the method/property from the object manager  68  (of  FIG. 3 ) in step  144 . The automation node passes information specifying the method/property to the object manager  68 . The object manager  68  returns the requested information. 
   The automation node receives the requested information from the object manager  68  in step  146 . Preferably, the requested information includes context for the method or object and a reference to a help file which includes the help screen to be displayed. In response to receiving the requested information, the automation node displays a help screen for the method/property, as shown in  FIG. 19 , in step  148 .  FIG. 19  shows a help screen for the Excel Application Workbooks method. It is noted that the same steps apply for a property as for a method even though the steps have been illustrated via screen shots with reference to a method. 
   Thus, the method provides a useful means for providing context sensitive help information for a user creating an automation client by means of the graphical programming environment. 
   In one embodiment, automation classes and methods and/or properties of the classes are dynamically specified. Preferably, the classes and methods are specified via a front panel control. The alternate embodiment is advantageous in circumstances wherein it is desired to access an automation server application which does not publish a type library. The type descriptor for the dynamically specified automation class and method and/or property embodiment includes an indication of a dynamic automation type. 
   Class Propagation 
   Referring now to  FIG. 20 , a flowchart illustrating steps taken to propagate the automation class of an automation node to another automation node is shown. In response to a user dropping a first automation node in a VI block diagram, the block diagram editor  64  invokes a method of the first automation node to display the first automation node in step  240 , as shown in FIG.  21 . In response to a user dropping a second automation node in a VI block diagram, the block diagram editor  64  invokes a method of the second automation node to display the second automation node in step  242 . 
   The first automation node is one of an automation open node, an automation invoke node, an automation property node, and an automation refnum. The second automation node is one of an automation open node, an automation invoke node, an automation property node, and an automation close node. Each of the automation nodes, has a refnum input terminal and a refnum output terminal for receiving and providing, respectively, a type descriptor which includes an automation class identifier and a type library identifier which the automation class is in. However, the automation refnum only has a refnum output terminal and the automation close node only has a refnum input terminal. 
   The first automation node has a first automation class and the second automation node has a second automation class. The first and second automation classes may be the same or different. The automation class of each of the first and second automation nodes is set either by default when the automation node is dropped or directly by the user, preferably using the “Select OLE Class” item in the automation node pop-up menu, as shown in FIG.  28 . 
   In response to a user wiring the first and second automation nodes together, the block diagram editor  64  displays a wire connecting the refnum output terminal of the first automation node to the refnum input terminal of the second automation node in step  244 . In response to the user wiring the first and second automation nodes together, the automation class of the first automation node is propagated to the second automation node in step  246 . That is, the second automation node receives the first automation class in the type descriptor from the first automation node and changes the automation class of the second automation node to the first automation class if the second automation class is different from the first automation class. 
     FIG. 22  shows first and second automation nodes prior to being wired together and  FIG. 23  shows the automation nodes after being wired together, i.e., after the automation class propagation has occurred. It is noted that the class of the second automation node is now the same as the class of the first automation node. 
   Referring now to  FIG. 24 , a flowchart illustrating steps taken to propagate the automation class of an automation node to another automation node is shown. In response to a user dropping a first automation node in a VI block diagram, the block diagram editor  64  invokes a method of the first automation node to display the first automation node in step  250 . In response to a user dropping a second automation node in a VI block diagram, the block diagram editor  64  invokes a method of the second automation node to display the second automation node in step  252 . 
   The first automation node is one of an automation open node, an automation invoke node, an automation property node, and an automation refnum. The second automation node is one of an automation open node, an automation invoke node, an automation property node, and an automation close node. Each of the automation nodes, has a refnum input terminal and a refnum output terminal for receiving and providing, respectively, a type descriptor which includes an automation class identifier and a type library identifier which the automation class is in. However, the automation close node only has a refnum input terminal and the automation refnum has either only a output terminal or an input terminal depending upon whether or not it is a control or an indicator, respectively. 
   The first and second automation nodes have a first automation class. The automation class of each of the first and second automation nodes is set either by default when the automation node is dropped or directly by the user, preferably using “Select OLE Class” item in the automation node pop-up menu, as shown in FIG.  28 . In addition, the automation class of the automation nodes may have previously been set via the class propagation steps described in the flowcharts of  FIG. 20  or FIG.  24 . 
   In response to a user wiring the first and second automation nodes together, the block diagram editor  64  displays a wire connecting the refnum output terminal of the first automation node to the refnum input terminal of the second automation node in step  254 . 
   In response to a user requesting to change the first automation node to a second automation class, the first automation node changes the automation node to be of the second automation class in step  256 . 
   In response to the user changing the first automation node to a second automation class, the automation class of the first automation node is propagated to the second automation node in step  258 . That is, the second automation node receives the second automation class in the type descriptor from the first automation node and changes the automation class of the second automation node to the second automation class if the second automation class is different from the first automation class. 
     FIG. 25  shows first and second automation nodes prior to changing the first automation node to be of the second automation class and  FIG. 26  shows the automation nodes after changing the first automation node to be of the second automation class, i.e., after the automation class propagation has occurred. It is noted that the class of the second automation node is now the same as the class of the first automation node. 
   Type Propagation Checking 
   Referring now to  FIG. 27 , a flowchart illustrating steps taken to perform type propagation checking of automation invoke nodes and automation property nodes is shown. In general, the method described in  FIG. 27  applies to both automation invoke nodes and automation property nodes, with differences noted. Hence, for clarity, an automation invoke node and an automation property node are referred to generically as an automation node with respect to  FIGS. 27 through 35 . Likewise, the method of an automation invoke node and the properties of an automation property node are referred to as method/property where either apply. 
   A user creates an automation node, i.e., an automation invoke node or an automation property node, in order to invoke methods/properties of an object instantiated from an automation class. Preferably, the user drags an automation node from the automation nodes palette of FIG.  5  and drops the automation node in a block diagram. 
   In response to the user dropping the automation node in the block diagram, the block diagram editor  64  invokes a method on the automation node to display an automation node icon in the block diagram, in step  200 . 
   The automation node displayed has an associated first automation class associated with it. In one case, the first automation class is a default automation class. In another case, the first automation class is selected by the user. In this case the user, preferably, pops up on the automation node and selects the first automation class from a list of previously selected automation classes in the pop-up menu as shown in  FIG. 28 , or from a list of the automation classes in the system as shown in FIG.  9 . In another case, the automation class of the automation node is propagated to the automation node from another automation node as described in  FIG. 20  or FIG.  24 . 
   In response to the user selecting a method/property for the automation node, the automation node selects the user-selected method/property to be invoked by the automation node in step  202 . The automation node displays the selected method/property in the automation node. 
   In step  204 , the class of the automation node is changed to be of a second automation class in response to user input. Preferably, the user input may be of three different types. 
   The first type of user input is the user wiring the refnum input terminal of the automation node to the refnum output terminal of second automation node, such as an automation open node, automation property node, or automation invoke node. The second automation node propagates its automation class, i.e., a second automation class, to the automation node, as described in the flowchart of FIG.  20 . The second automation class may or may not be different from the first automation class, i.e., from the automation class of the automation node prior to the wiring. 
   The second type of user input is changing the automation class of a second automation node, whose refnum output terminal is already wired to the refnum input terminal of the automation node, from the first automation class to the second automation class. As in the first type of user input, the second automation node propagates its automation class, i.e., the second automation class, to the automation node, as described in the flowchart of FIG.  24 . 
   The third type of user input is the user selecting the second automation class from a list of automation classes directly, preferably, via either the pop-up menu of the automation node as shown in  FIG. 9 , or the browse list as shown in FIG.  10 . When the user selects a new automation class from one of the two lists of classes, the automation node changes its class to the newly selected second automation class. 
   In response to the user changing the automation node to be of the second automation class, the automation node performs type propagation checking to determine if the method/property is valid for the second automation class, in step  206 . The automation node makes this determination in step  208 . Performing the type propagation checking of step  206  will be described in more detail below with reference to FIG.  31 . 
   If the method/property of the automation node is valid for the second automation class, the block diagram editor  64  displays an indication of a valid condition, in step  214 . 
   In the case that the automation node is an automation invoke node, for the first and second types of user input, the indication of a valid condition preferably includes displaying a valid wire. In one embodiment, a valid wire is a solid wire. For the third type of user input, the indication of a valid condition preferably includes displaying at least a portion of the automation invoke node in a color which indicates a valid condition. In one embodiment, a color which indicates a valid condition includes a color for each terminal of the method which indicates the data type of the terminal, wherein the color is not a color which indicates an invalid condition. In one embodiment, the color which indicates an invalid condition is black. 
   In the case that the automation node is an automation property node the indication of a valid condition preferably includes displaying the property in a color indicative of a valid condition. In one embodiment, a color which indicates a valid condition includes a color for each property which indicates the data type of the property, wherein the color is not a color which indicates an invalid condition. In one embodiment, the color which indicates an invalid condition is black. 
   If the method/property of the automation node is not valid for the second automation class, the block diagram editor  64  displays an indication of an invalid condition, in step  210 . 
   In the case that the automation node is an automation invoke node, for the first and second types of user input, the indication of an invalid condition preferably includes displaying a broken wire. In one embodiment, a broken wire is a dashed wire, as shown in FIG.  29 . For the third type of user input, the indication of an invalid condition preferably includes displaying at least a portion of the automation invoke node in a color which indicates an invalid condition. In one embodiment, a color which indicates an invalid condition is black. 
   In the case that the automation node is an automation property node the indication of an invalid condition preferably includes displaying at least a portion of the automation property node in a color which indicates an invalid condition. In one embodiment, a color which indicates an invalid condition is black. 
   In the case where the automation node is an automation property node, the type propagation includes performing steps  208 ,  210  and  214  for each of the properties of the automation property node. 
   If the method/property of the automation node is not valid for the second automation class, the block diagram editor  64  further displays an error message in an error window, as shown in  FIG. 30 , in step  212 . In one embodiment, if the method/property of the automation node is not valid for the second automation class, the graphical programming environment prevents execution of the virtual instrument, in step  214 . That is the graphical programming environment prevents the automation node from invoking the associated method/property. 
   Referring now to  FIG. 31 , a flowchart illustrating in more detail the step  206  of  FIG. 27  of performing type propagation is shown. In response to the user changing the automation node to be of the second automation class, the automation node passes information to the object manager  68  (of  FIG. 3 ) in step  220 . The information passed includes information identifying the method/property of the automation node, information identifying the second automation class, and information identifying a type library, of the type libraries  70  (of FIG.  3 ), which the second automation class is in. Preferably, the automation node invokes a function of the object manager  68  which returns a value indicating whether or not the method/property is a valid method/property for the second automation class, i.e., whether or not the second automation class defines the method/property. Preferably, the automation node receives the information in a type descriptor, described above, via the refnum input terminal of the automation node. 
   In response to the automation node passing the information to the object manager  68 , the object manager  68  requests a list of valid methods/properties of the second automation class from the type library specified in the passed information in step  222 . The type library includes information specifying the methods and properties defined by each class in the type library. 
   The type library returns the requested list and the object manager  68  receives the requested list from the type library and creates a data structure including the list of valid methods/properties of the second automation class in step  224 . 
   In one embodiment, the object manager  68  maintains the data structure created and determines in response to step  220  whether or not the list of valid methods/properties exists. If the list already exists, the object manager  68  does not perform step  222 . 
   The object manager  68  searches the list of valid methods/properties in the data structure in step  226  and determines if the method/property is valid, i.e., present in the list, in step  228 . If the specified method/property is present in the list of valid methods/properties, the object manager  68  returns a value indicating a valid method/property in step  230 . If the specified method/property is not present in the list of valid methods/properties, the object manager  68  returns a value indicating an invalid method/property in step  232 . 
     FIGS. 32 ,  33  and  34  are screen shots illustrating a help screen for an automation open node, an automation property node, and an automation invoke node, respectively. The help screens illustrate the various terminals of the automation nodes. 
   Although the system and method of the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents, as can be reasonably included within the spirit and scope of the invention as defined by the appended claims.