Abstract:
The subject disclosure pertains to systems and methods for supporting null capabilities for general purpose programming languages. Low level, runtime support for a nullable type is provided to ensure consistency and coherency. An execution component restricts support of the nullable type to programming language types that lack a null capability. The execution component may also provide support for conversion between the nullable type and a reference type.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to co-pending U.S. application Ser. No. 10/935,247, entitled, “GENERAL PROGRAMMING LANGUAGE SUPPORT FOR NULLABLE TYPES”, filed on Sep. 7, 2004. The entirety of the aforementioned application is incorporated herein by reference. 
   BACKGROUND 
   As programming approaches and foundations have evolved, application programming interfaces (APIs) and programming schemas have been developed to standardize and unify programming methodologies that were previously multi-variant and relatively incompatible. Modem programming therefore often involves employing APIs and schemas in conjunction with reusable libraries. Unfortunately, this evolution has inadvertently created many inefficient programming artifacts that are actually spread and perpetuated by these standardizations programming constructs. For example, one resulting software programming artifact is a duality between reference types and value types. 
   Reference types are variables that are stored on a heap and referenced by a pointer stored on the stack. Value types are variables that are stored directly on the stack. Consequently, variables that are represented as reference types can be uninitialized (termed “null”), but variables that are represented as value types cannot be established in an uninitialized condition without risking indeterminate or even catastrophic results. This nullification issue can present problems in a myriad of situations, including data base accessing. 
   Support for nullability across all types, including value types, is essential when interacting with null supported languages, such as database languages (e.g., SQL). A database language may use a null value to indicate that the actual data value is unknown or missing. By providing null support within a general purpose programming language null-supported languages and programming languages may be integrated (e.g., code in the C# programming language may read, write or interpret nullable fields in a database language). 
   General purpose programming languages have historically provided little or no null support. Many approaches exist for handling nulls and value types without direct language support, but all have shortcomings. For example, one approach is to use a “special” value (such as −1 for integers) to indicate null, but this only works when an unused value can be identified. Another approach is to maintain Boolean null indicators in separate fields or variables, but this does not work well for parameters and return values. A third approach is to use a set of user-defined nullable types, but this only works for a closed set of types. 
   Certain general purpose programming languages supply a nullable type (e.g., C# 2.0) to provide for nullability across all types, including value types. A nullable type may be implemented using a multi-element structure containing a Boolean null indicator and an element which contains the value of the instance of the nullable type when the instance of the nullable type does not represent null. Although the nullable type provides a method for setting a value type to null, the structure used to implement the nullable type is itself an instance of a value type. Use of a structure that is inherently an instance of a value type to represent an instance of a reference type leads to inconsistent and counterintuitive results under certain circumstances. 
   SUMMARY 
   The following presents a simplified summary in order to provide a basic understanding of some aspects of the claimed subject matter. This summary is not an extensive overview. It is not intended to identify key/critical elements or to delineate the scope of the claimed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. 
   Briefly described, the provided subject matter concerns null support for general purpose programming language types. Typically, support is provided using a nullable type including an underlying value and an indicator value. The indicator value indicates whether the value of the instance of the nullable type represents a null. In one aspect of the subject matter disclosed herein, low-level, runtime support for the nullable type is provided. This low-level support increases coherency. 
   The disclosed subject matter provides for the conversion between the nullable type, which is a value type, and reference type. An instance of the nullable type is unwrapped to allow access to the elements of the instance of the nullable type. The elements, the underlying value and the indicator value, are used to create an instance of a reference type. 
   Support of nullable at the runtime level eliminates seeming inconsistencies. Elimination of these inconsistencies increases programming language predictability and usability for software developers. In addition, the provision of support for the nullable type at a low-level allows for use of the nullable type in multiple programming language environments. 
   To the accomplishment of the foregoing and related ends, certain illustrative aspects of the claimed subject matter are described herein in connection with the following description and the annexed drawings. These aspects are indicative of various ways in which the subject matter may be practiced, all of which are intended to be within the scope of the claimed subject matter. Other advantages and novel features may become apparent from the following detailed description when considered in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a system for providing nullable type support for programming languages. 
       FIG. 2  is a block diagram of a nullable type. 
       FIG. 3  is a block diagram of a system for compiling source code including the nullable type. 
       FIG. 4A  is a block diagram of a runtime system that provides for the processing of nullable types. 
       FIG. 4B  is a block diagram of an integrated software development environment that provides for the processing of nullable types. 
       FIG. 5  is a flowchart diagram of a boxing instruction methodology. 
       FIG. 6  is a flowchart diagram of an unboxing instruction methodology. 
       FIG. 7  illustrates a box instruction relationship for nullable types. 
       FIG. 8  illustrates an unbox instruction relationship for nullable types. 
       FIG. 9  is a block diagram of an integrated development environment. 
       FIG. 10  is a schematic block diagram illustrating a suitable operating environment. 
       FIG. 11  is a schematic block diagram of a sample computing environment. 
   

   DETAILED DESCRIPTION 
   The various aspects of the subject matter are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. It may be evident, however, that the subject matter may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the claimed subject matter. 
   As used in this application, the terms “component” and “system” are intended to refer to a computer-related entity, either hardware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer. By way of illustration, both an application running on a server and the server can be a component. One or more components may reside within a process and/or thread of execution and a component may be localized on one computer and/or distributed between two or more computers. 
   The word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. In addition, while the examples provided utilize the C# programming language, numerous alternative programming languages may be used (e.g., C, C++, C#, Pascal, Python, Ruby, Visual Basic, Java, and so forth). 
   As used herein, the terms to “infer” or “inference” refer generally to the process of reasoning about or inferring states of the system, environment, and/or user from a set of observations as captured via events and/or data. Inference can be employed to identify a specific context or action, or can generate a probability distribution over states, for example. The inference can be probabilistic—that is, the computation of a probability distribution over states of interest based on a consideration of data and events. Inference can also refer to techniques employed for composing higher-level events from a set of events and/or data. Such inference results in the construction of new events or actions from a set of observed events and/or stored event data, whether or not the events are correlated in close temporal proximity, and whether the events and data come from one or several event and data sources. 
   Referring initially to  FIG. 1 , a system  100  provides a nullable type support for programming languages in accordance with an aspect of the claimed subject matter. The system  100  includes one or more data structures  110  that can be declared in the context of a programming language  120  such as C, C++, C#, Pascal, Python, Ruby, Visual Basic, Java, and so forth. The data structures  110  generally consist of an underlying type such as an integer, float, Boolean, and so forth that can be declared to be a nullable type by associating a type modifier with the underlying type. For instance, a symbol such as “?” can be employed as a type modifier to indicate to a compiler component  130  that the underlying type is to be considered a nullable type. It is to be appreciated that various other symbols can be employed as the type modifier. Alternatively, an underlying type may be declared a nullable type by associating the word nullable with the underlying type (e.g., Nullable&lt;int&gt;). The expressions T? and nullable&lt;T&gt;, where T is a programming language type, may be used interchangeably herein. Upon declaration and compilation, implementation code  140  (e.g., executable, intermediate language . . . ) can be generated via the compiler component  130  including nullable and non-nullable types, if desired by the software developer. 
   Referring now to  FIG. 2 , an instance of a nullable type may be set to any value available for the underlying type and the null value. A nullable type  200  may include an underlying value  210  containing the value of the underlying type and an indicator value  220 . The indicator value signals when the value of the instance of the nullable type is null. An instance of the nullable type may be implemented by automatically creating a multi-element structure including an element for the underlying value  210  and a Boolean element for the indicator value  220 , representing whether or not the underlying value is null (e.g., flag indicating null or non-null value for the underlying type). If the Boolean element is declared false for example, the underlying type is considered null, whereas if the Boolean element is true for example, the value contained in the underlying value is considered the value of the instance of the nullable type. 
   Generally, nullable types are constructed using the ? type modifier (or other symbol). For example, int? is a nullable form of the predefined type int. A nullable type&#39;s underlying type should be a non-nullable value type. Consider the following exemplary declaration of the nullable type: 
                                           public struct Nullable&lt;T&gt; where T : struct            {            internal T value;            internal bool hasValue;            public Nullable(T value) {             this.value = value;             this.hasValue = true;            }            public bool HasValue {             get {              return hasValue;             }             }             public T Value {              get {              if (!hasValue) {               throw new InvalidOperationException( );              }              return value;              }             }            }                        
As shown in the exemplary code, an instance of the nullable type may have two public read-only properties, value and hasValue. The property hasValue is of type bool and signals that the nullable type has a value of null. For example, hasValue is true for a non-null instance and false for a null instance. The property value is of the nullable type&#39;s underlying type T. The nullable type includes two methods Value and HasValue that can be used to test the null status and retrieve the value. The method HasValue returns the hasValue property. When hasValue is true, the Value method returns the contained value. When hasValue is false, an attempt to access the value property throws an exception.
 
   The exemplary nullable type also includes a public constructor that takes a single argument of type T. The constructor initializes an instance of a nullable type. For a non-null instance of T? the value property is set to the value of T and the hasValue property is set to true. The constructor for the nullable type should not permit nesting of nullable types. The value property should not be set to null. 
   In general, a non-nullable value type may be implicitly converted to a nullable form of that value type. Furthermore, an implicit conversion can exist from the null literal to any nullable type. For example: 
                                           int? x = 123;           int? y = null;           if (x.HasValue) Console.WriteLine(x.Value);           if (y.HasValue) Console.WriteLine(y.Value);                        
In the first two lines of the above example, the integer value 123 and the null literal are implicitly converted to the nullable type int?. The example code would result in the first Console.WriteLine executing for x and outputing 123. However, the second Console.WriteLine would not execute since y.HasValue would return false.
 
   The type specified before the ? modifier or within the “&lt;&gt;” for a nullable type is referred to as the underlying type of the nullable type. The underlying type of a nullable type can be any non-nullable value type. Typically the underlying value cannot be a reference type or a nullable type. For example, int? and double? are valid nullable types, but string? and int?? are generally not. A nullable type can represent all values of its underlying type plus an additional null value. The syntax T? is shorthand for System.Nullable&lt;T&gt;, and the two forms can be used interchangeably. 
   The term wrapping, as used herein, denotes the process of packaging a value of type T, in an instance of a type of the form nullable&lt;T&gt;. Conceptually, a value x of type T is wrapped to type T? by evaluating the expression new T?(x). The term unwrapping denotes the process of obtaining the value of type T, contained in an instance of a type of the form nullable&lt;T&gt;. Conceptually, a value x of type T? is unwrapped to type T by evaluating the expression x.value. Attempting to unwrap a null instance may cause an exception to be thrown. 
   Certain programming languages provide a bridge or method for conversion between value types and reference types. For example, C# provides the boxing and unboxing instructions to convert between value types and reference types. A boxing conversion allows any value type to be implicitly converted to a reference type. Boxing an instance of a value type converts the value type instance to an instance of the object class type; the object class type is the ultimate base class of all other types in C#. Although the following examples are presented using C# any subject matter disclosed is not limited to the C# programming language. Consider the following C# code: 
                                           int i = 123;           object box = i;                        
Here, a value type variable, int i, is declared and set to a value of 123. An object type variable box is declared and i is boxed, or implicitly converted to the object type. Conversely, an unboxing conversion allows a reference type to be explicitly converted to a value type. For example:
 
                                           object box = 123;           int i = (int) box;                        
Here, in the first line of the sample code, an object type variable box is declared and set to a value of 123. In the second line, an int i is declared. The value of box is unboxed or converted to an int and i is set to the value of box.
 
   Unfortunately, boxing and unboxing of nullable type variables may cause inconsistencies. Although the nullable type provides a null value, the nullable type is in fact a value type variable. As illustrated above, the nullable type may be implemented as a multi-element structure. Therefore, when the nullable type, nullable&lt;T&gt; is boxed, the resulting reference type is not a null reference. Because the compiler treats the nullable&lt;T&gt; as a structure, boxing the nullable&lt;T&gt; structure results in a non-null reference to the boxed nullable&lt;T&gt;. This causes inconsistencies when the nullable&lt;T&gt; contains a null value. Consider the following exemplary C# code: 
                                           Nullable&lt;int&gt; x = null;           Console.WriteLine(x == null); // True since HasValue is false           object o = x;           Console.WriteLine(o == null); // False since o is a proper           reference to “null” value x                        
Here, a nullable type variable x, with an underlying type int, is set to null. Using the declaration of nullable provided above, the nullable constructor sets the Boolean property hasValue to false. Therefore, the comparison of x to null in the second line is equal to true. However, in the third line, when x is implicitly boxed to object o, the object variable o contains a reference to the null value x, rather than containing an actual null reference. Therefore, when the object variable o is compared to null in the fourth line of the example code, the comparison will return a value of False. Consequently, the result of the comparison differs depending upon whether the comparison is made using the nullable type variable or the boxed nullable type variable. This result may be counterintuitive to software developers. The problem is also apparent when an unconstrained type parameter is used rather than a type object. For example:
 
                                           void F&lt;T&gt;(T x) { Console.WriteLine(x == null); }           int? x = null;           F(x); // False                        
Here, a nullable type variable x, with an underlying type of int, is set to null. The function F is called with parameter x. The unconstrained type parameter x is boxed and the result of the comparison of unconstrained type parameter is False.
 
   As illustrated in the example above, for a value x of a type T, the chain of implicit conversions from type T to type nullable&lt;T&gt; to type object results in a different value than a conversion directly from type T to type object. This result leads to incoherence. Now, consider the following C# code: 
                                           int x = 123;           object o = x;           int y = (int?)o; // Error, must write (int?)(int)o                        
Here, an int type variable x is declared and set to a value of 123. The variable x is then boxed to an object type variable o. The third line attempts to unbox the object type to a nullable type, with an underlying integer value, and then box the nullable type to an int. However, the code as written would likely generate an error. A boxed T cannot be unboxed as nullable&lt;T&gt; and a boxed nullable&lt;T&gt; cannot be unboxed as a T. Ideally, software developers should be able to box and unbox T and nullable&lt;T&gt; interchangeably.
 
   In addition, the nullable of type T does not typically implement the interfaces of the underlying type T. In general, an interface defines a set of properties, methods and events, but does not provide implementation. In particular, in C# an interface type is a partial specification of a type. All classes that inherit from the interface type must implement the methods contained in the interface type. For example: 
                                           struct S: IFoo {...}           S? x = new S( );           IFoo foo = x;    // Compile-time error           IFoo foo = (IFoo)x;   // Compile-time error           IFoo foo = (IFoo)(object)x; // Run-time error           IFoo foo = x.HasValue ? (IFoo)x.Value : null;                        
A struct S inherits from an interface IFoo. A nullable type variable x with an underlying type S is instantiated. The interface method IFoo is accessible only through the value element, which is of the underlying type S, of the nullable type, as shown in the last line of the exemplary code. In addition, the interface is only accessible if the nullable type variable x is not null. Consequently, only the last line of code properly utilizes the interface IFoo of struct S.
 
   The conventional implementation of the nullable type also allows nesting of nullables. In general, the nullable type should be utilized only with underlying types that are not capable of expressing nullability themselves. Generally, taking the nullable of a reference type may be prevented by adding a constraint to the T on the nullable&lt;T&gt; declaration. However, this constraint would not prevent a software developer from creating nested nullable types (e.g., nullable&lt;nullable&lt;T&gt;&gt;) because nullable&lt;T&gt; is itself a structure and therefore a value type variable. 
     FIG. 3  illustrates a system  300  for implementing source code including the nullable type. The source code  310  may be in any programming language that includes the nullable type. The source code  310  is processed by a compiler component  320  Compilation produces the implementation code  330 . 
     FIG. 4A  illustrates a runtime system  400  that provides for the processing of nullable types. A runtime component  402  may include an identifier component  404  and a conversion component  406 . The identifier component  404  identifies instances of the nullable type. The conversion component  406  handles conversion to and from the nullable type. The identifier component  404  and conversion component  406  may be implemented by a runtime compiler or an execution engine or a combination thereof. 
     FIG. 4B  illustrates an integrated software development framework  450  (e.g. NET) that provides for the processing of nullable types. The framework  450  may include one or more programming language compiler components  402  (e.g., C#, VB.Net). These compiler components  452  may be used to generate an intermediate language code (e.g., MSIL). The intermediate language code may be processed by a runtime environment,  454  (e.g., CLR) also referred to herein as the runtime component. The runtime environment  454  may include a runtime compiler component or Just In Time compiler  456  (JIT) and an execution engine  458  to process the intermediate language code. The software development framework  450  may include a set of common libraries  460 , such as the System namespace. The common libraries  460  may include a reflection class  462  and serializer class  464 . Full support of the nullable type may require modification of several of the elements of the software development framework  450 .The above discussed inconsistencies of the nullable type may be resolved by providing deep support for the nullable type. Low-level support may be provided in the runtime environment (e.g., Common Language Runtime (CLR), Java Virtual Machine (JVM)). Low-level, execution component support for nullable types may be used to provide nullable types in a multi-language environment. Although the .Net framework and CLR runtime environment are discussed here in detail, support may be provided any runtime environment. 
   The low level nullable type support should recognize structures capable of representing the null value. Recognition of structures which support the null value would allow a runtime distinction between nullable types and other structures. Once the nullable type may be differentiated from other structures, including programmer defined structures, the nesting of nullable types may be prevented. Alternatively, the compiler may not allow structures to be converted into nullable types. The execution may include a special verifier rule that interprets the struct constraint as “cannot represent null”, identifying the variable as a struct, but not a nullable type. 
   Referring now to  FIGS. 5 and 6 , the runtime environment may be modified to properly convert between a nullable type and a reference type (e.g., box and unbox the nullable type). In particular, in one aspect, the runtime compiler component, JIT, may be modified to provide the specialized box and unbox instructions.  FIG. 5  illustrates a boxing instruction that recognizes and unwraps a nullable type, for example nullable&lt;T&gt;. Nullable&lt;T&gt; is mapped it to a null pointer if the value of nullable&lt;T&gt; is null or to a boxed, unwrapped T if the value of nullable&lt;T&gt; is not null. Beginning at  502 , the box instruction is received. At  504 , it is determined whether the variable to be boxed is a nullable type. If the variable is not a nullable type, the processing continues with the conventional boxing instruction at  506 . If the variable is a nullable type, at  508  it is determined whether the value of the nullable is null. If yes, the box instruction will return a null pointer at  510 . If no, the box instruction will unwrap the nullable&lt;T&gt; and box the instance of type T contained within the nullable&lt;T&gt;. Thus, boxing a non-null nullable&lt;T&gt; results in a boxed T. This boxing operation may be thought of as lossy. The result of boxing a nullable&lt;T&gt; stores no information to indicate that it was originally a nullable&lt;T&gt; rather than a T. The boxed representation of a nullable&lt;T&gt; is either a null reference or a reference to a boxed&lt;T&gt;. Consequently, there is no dynamic type nullable&lt;T&gt; and the heap never holds a nullable&lt;T&gt;. When the nullable&lt;T&gt; is boxed, it is unwrapped and the underlying value is stored on the heap, rather than the nullable&lt;T&gt;. 
   In one aspect of the disclosed subject matter, a true static boxed&lt;nullable&lt;T&gt;&gt; may still exist in the runtime. The execution engine may allocate and manage static fields on the managed heap. The static fields should be fully self-describing and therefore boxed. To avoid losing type information, a true boxed&lt;nullable&lt;T&gt;&gt; is maintained and the unbox operation will succeed in unboxing the boxed&lt;nullable&lt;T&gt;&gt; into a nullable&lt;T&gt;. 
     FIG. 6  illustrates a process for unboxing a nullable type, where a variable of type boxed&lt;T&gt; may be unboxed to either a T or a nullable&lt;T&gt;. Beginning at  602 , an unboxing instruction is received. At  604  a determination is made as to whether the variable is to be unboxed to a nullable type. If no, conventional unboxing is performed at  606 . If yes, a determination is made as to whether the value of the variable is null at  608 . If yes, the hasValue element of the nullable&lt;T&gt; is set to false and the value element is set to the default value for T at  610 . If no, the hasValue element of the nullable&lt;T&gt; is set to true at  612  and the value element is set to the value of the variable at  614 . 
     FIG. 7  illustrates the relationship  700  of the box instruction to a type T  710 , the object type  720  and a nullable&lt;T&gt;  730 . As discussed above, without low level support for nullable types boxing a variable of type T  710  to an object type  720  may yield different results than boxing a nullable&lt;T&gt;  730  to an object type  720 . As shown in  FIG. 7 , when runtime support is provided for nullable types and the box instruction is modified as discussed above, boxing a Type T  710  to an object type  720  is equivalent to taking a nullable of type T  710  and then boxing the nullable&lt;T&gt;  730  to an object type  720 . Therefore, a box of Type T is equivalent to a box of nullable&lt;T&gt;. 
     FIG. 8  illustrates the relationship  800  of the unbox instruction to the object type  810 , a type T  820  and a nullable&lt;T&gt;  830 . Now a boxed T, which is an object type  810 , may be unboxed either to a type T  820  or to a nullable&lt;T&gt;  830 . If the value of the object type boxed T  810  is null, unboxing to a nullable&lt;T&gt;  830  succeeds, yielding a nullable&lt;T&gt;  830  with a null value. If a null object type is unboxed to a type T, an exception is thrown. 
   To summarize, with these modifications to the box and unbox instructions it would be effectively impossible to have a boxed nullable&lt;T&gt;. After all, when nullable&lt;T&gt; is boxed, only type T is stored in the object type, or box. Therefore, a type T would have only one boxed form. In addition, a null value of type nullable&lt;T&gt; would always map onto a null reference. For a non-null of type nullable&lt;T&gt;, the value element is unwrapped and then boxed, resulting in boxed &lt;T&gt;. A boxed instance of T could be unboxed either as a T (which would throw an exception if boxed T was equal to null) or as a nullable&lt;T&gt; (which would map a null reference to a null value). The modifications of the box and unbox instructions would eliminate the confusion around boxed T and boxed nullable&lt;T&gt;. 
   In addition, the modification of the box and unbox instructions would make nullable&lt;T&gt; appear to implement T&#39;s interfaces when it is boxed. As discussed above, the modified boxing instruction effectively unwraps the nullable&lt;T&gt;. Accordingly, boxed nullable&lt;T&gt; is equivalent to boxed T and therefore has access to any interfaces of type T. However, it is important that all of the interfaces implemented by nullable&lt;T&gt; be implemented by type T. If type T does not have all of the interfaces, then when a nullable&lt;T&gt; is unwrapped and boxed as a type T, some of the interface implementations may be lost. One possible method for ensuring that T implements all of the interfaces of nullable&lt;T&gt; is to constrain T. Consider the following exemplary C# code using generic constraints: 
                                           public struct Nullable&lt;T&gt;:            IComparable,            IComparable&lt;Nullable&lt;T&gt;&gt;,            IEquatable&lt;Nullable&lt;T&gt;&gt;           where T: struct,            IComparable,            IComparable&lt;T&gt;,            IComparable&lt;Nullable&lt;T&gt;&gt;,            IEquatable&lt;T&gt;,            IEquatable&lt;Nullable&lt;T&gt;&gt;           {            ...           }                        
Type T is constrained to implement IComparable, and IEquatable. Unfortunately, this limits the usefulness of nullable&lt;T&gt;. Effectively, a struct would have to be specifically authored to support use with nullable&lt;T&gt;. Alternatively, the CLR could automatically implement any missing interfaces for a struct.
 
   To prevent the interface problems discussed above, the nullable&lt;T&gt; may be implemented without interfaces. The default comparers and equitable comparers within the runtime environment (e.g., CLR) may be altered to recognize nullable&lt;T&gt;s. Consider the following exemplary code: 
                                            internal class NullableComparer&lt;T&gt; : Comparer&lt;Nullable&lt;T&gt;&gt;           where T : struct, IComparable&lt;T&gt;            {             public override int Compare(Nullable&lt;T&gt; x, Nullable&lt;T&gt; y) {              if (x.hasValue) {               if (y.hasValue) return x.value.CompareTo(y.value);               return 1;              }              if (y.hasValue) return −1;              return 0;             }            }                        
Here, a class NullableComparer may be added to the Comparer implementation to provide for nullable types. Similarly, the following exemplary code may be added to the EqualityComparer implementation:
 
   
     
       
             
             
           
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
         
             
                 
                 
             
           
           
             
                 
               internal class NullableEqualityComparer&lt;T&gt; : 
             
           
        
         
             
               EqualityComparer&lt;Nullable&lt;T&gt;&gt; where T : struct, IEquatable&lt;T&gt; 
             
           
        
         
             
                 
               { 
             
           
        
         
             
                 
               public override bool Equals(Nullable&lt;T&gt; x, 
             
             
                 
               Nullable&lt;T&gt; y) { 
             
           
        
         
             
                 
               if (x.hasValue) { 
             
           
        
         
             
                 
               if (y.hasValue) return x.value.Equals(y.value); 
             
             
                 
               return false; 
             
           
        
         
             
                 
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               if (y.hasValue) return false; 
             
             
                 
               return true; 
             
           
        
         
             
                 
               } 
             
             
                 
               .... 
             
             
                 
               } 
             
           
        
         
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   Additional modifications to the runtime environment may be required to support full nullable functionality. The runtime environment may need to provide instructions that allow inspection of the nullable type at runtime. For example, runtime instructions that test for assignment compatibility of an object with a type token T (e.g., the isinst instruction of the JIT provided by CLR) may need to be updated. The isinst instruction must have intrinsic knowledge of the nullable type to perform type checking. The isinst should leave a reference to the object on the stack to indicate success. Otherwise, a null reference is left on the stack. When the isinst instruction is successful, unboxing is not performed. The code will perform an unbox into a nullable&lt;T&gt; slot if necessary. 
   The modified runtime instruction to test for type compatibility may be utilized by general purpose programming language operators. For example, the C# operators “is” and “as” utilize the isinst operation. Accordingly, the “is” and “as” operators may be extended to handle any type that permits nullability (e.g., reference types and nullable types). 
   The “is” operator is used to dynamically check if the runtime type of an expression is compatible with a given type. The result of an “is” operation, “e is T”, where e is an expression and T is a type, is a Boolean that indicates whether the expression may be converted to the type T by a reference conversion, a boxing conversion or an unboxing conversion. The “is” operation may need to be modified, such that the expressions “e is T?” evaluates to True when e is T or a non-null T? and False otherwise. In addition, the expression “e is T” should evaluate to True when e is T or a non-null T? and should be False otherwise. 
   Consider the following exemplary code: 
                               int? x = null;       int? y = 10;       int z = 10;       bool b1 = (x is int?); // False; x is boxed as null, null is always false       bool b2 = (y is int?); // True; y is boxed as int, new behavior ensures this       is true       bool b3 = (z is int?); // True; z is boxed as int, same as b2 (can&#39;t       differentiate)       bool b4 = (y is int) &amp;&amp; (z is int); // True; y and z are boxed as int,       ordinary check applies                    
Here, a nullable type variable x, with an underlying type of int, is initialized to null. A nullable type variable y, with an underlying type of int, is initialized to 10. An int type variable z is also initialized to 10. As indicated above in the fifth line of the sample code, b1 will be set to false. The variable x is a nullable type, with a null value. Taking a nullable of a null value is not permitted. Therefore, x may not be converted to a nullable of x and b1 is set to False. In contrast, b2 will be set to true in the sixth line of the sample code. Here, the nullable type variable y is set to 10. When the nullable y is boxed, the result will be the value 10. Taking a nullable of int 10 is permitted and consequently b2 is set to True. Example b3 in the seventh line is similar to b2, in that both y and z box to an int and therefore are nullable. Accordingly, b3 is also set to True. Finally, nullable type variable y has a value 10 and may therefore be boxed to an int. Consequently, b4 is set to True.
 
   The “as” operator is used to explicitly convert a value to a given reference type using a reference conversion or a boxing conversion. If the conversion is not possible, the resulting value is null. In an operation of the form “e as T”, e must be an expression and T must be a reference type. To fully support the nullable type, the “as” instruction may need to be modified such that for the operation “e as T” T must be a reference type or a nullable type. 
   In addition, the runtime environment may provide a mechanism for reifying class information at runtime. As used herein, reifying runtime values allows developers to view and manipulate runtime values. For example, the .NET framework includes Reflection. Reflection is provided through a set of base classes in the System.Reflection namespace and allows developers to view and manipulate values in the CLR using an object model. Reflection may be used to programmatically call methods, inspect objects and determine the available methods and members of certain objects. Because Reflection views and manipulates data types it requires information regarding available types, including the nullable type. Although this has been discussed particularly with the .Net and Reflection, any such mechanism should have the ability to handle the nullable type. 
   The set of Reflection APIs provided to the programmers uses a series of function calls (fcalls) RuntimeMethodHandle._InvokeMethodFast, _ConstructorInvoke, and _SerializationInvoke (e.g. Type.InvokeMember, MethodInfo.Invoke, ConstructorInfo.Invoke, etc.). These core fcalls may need to be modified to implement the correct semantics for unwrapping nullable types to deliver arguments to methods that expect real nullable values. Given a boxed argument which is of type boxed&lt;T&gt; or null, the fcall must successfully bind to parameters of type nullable&lt;T&gt;. The Reflection APIs accept object types as input, accordingly, there is no way to get the raw nullable&lt;T&gt; through to the implementation and fcalls. This creates two separate problems: (i) deciding whether the provided arguments correctly match a method&#39;s parameters; and (ii) performing conversions from null or type T to a nullable&lt;T&gt; when dispatching to a method that takes a nullable&lt;T&gt; as an argument. For example, if you have a method handle for an overload “void foo(int? x)”, the code which invokes foo may pass ‘x’ inside an object array. The act of creating the array will box the argument, resulting in either a boxed int or a null. The reflection code must know to either wrap or unwrap the argument before delivering it to ‘foo’. Consider the following exemplary code: 
                               int? x = null;       int? y = 10;       MethodInfo m = /*...*/;       object target = /*...*/;       //Reflection sees a boxed&lt;int&gt; being passed to something which takes       a Nullable&lt;int&gt;       m.Invoke(target, new object[ ] { x });       //Reflection sees a null being passed to something which takes a       Nullable&lt;int&gt;       m.Invoke(target, new object[ ] { y });                    
If the programming language has its own late-binding code (e.g., VB.NET), the programming language may have to handle this problem as well.
 
   In addition, any serializers or deserializers provided by the software development framework may need to be updated to provide deep understanding of the nullable type. Serialization is the process of taking an object and converting it into a format in which it can be transmitted across a network and saved to a storage location. Accordingly, any serializers in the runtime environment may need to have deep knowledge of nullable&lt;T&gt;. The representation of a boxed nullable&lt;T&gt; may confuse a serializer (e.g., when the serializer would like to serialize the raw nullable structure containing the Boolean hasValue and value). 
   Referring now to  FIG. 9 , for each programming language that includes the nullable type, certain applications may need to be modified to fully support the nullable type. An integrated development environment (IDE)  900  is a set of programs or applications run from a single user interface to provide a set of tools for software development. For example, an IDE  900  often includes a text editor component  910 , compiler component  920 , debugger component  930  and designer component  940 , which are all activated and function from a common user interface. A designer component  940  may facilitate the design and coding of a graphical user interface. The IDE  900 , including, but not limited to, the compiler component  920  and debugger component  930  may be modified to support the low level understanding of the nullable type. For example, existing debuggers may not understand that the new physical representation of a nullable&lt;T&gt; is either a boxed&lt;T&gt;, or a null reference. 
   The subject matter disclosed herein may be implemented as a method, apparatus, or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof to control a computer to implement the disclosed subject matter. The term “article of manufacture” (or alternatively, “computer program product”) as used herein is intended to encompass a computer program accessible from any computer-readable device, carrier, or media. For example, computer readable media can include but are not limited to magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips . . . ), optical disks (e.g., compact disk (CD), digital versatile disk (DVD) . . . ), smart cards, and flash memory devices (e.g., card, stick). Additionally it should be appreciated that a carrier wave can be employed to carry computer-readable electronic data such as those used in transmitting and receiving electronic mail or in accessing a network such as the Internet or a local area network (LAN). Of course, those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope or spirit of the claimed subject matter. 
   Referring to  FIGS. 5 and 6 , methodologies relating to supporting nullable types are illustrated. While, for purposes of simplicity of explanation, the methodologies are shown and described as a series of acts, it is to be understood and appreciated that the methodologies are not limited by the order of acts, as some acts may, in accordance with these methodologies, occur in different orders and/or concurrently with other acts from that shown and described herein. For example, those skilled in the art will understand and appreciate that a methodology could alternatively be represented as a series of interrelated states or events, such as in a state diagram. Moreover, not all illustrated acts may be required to implement the following methodologies. 
   In order to provide a context for the various aspects of the disclosed subject matter,  FIGS. 10 and 11  as well as the following discussion are intended to provide a brief, general description of a suitable environment in which the various aspects of the disclosed subject matter may be implemented. While the subject matter has been described above in the general context of computer-executable instructions of a computer program that runs on a computer and/or computers, those skilled in the art will recognize that the claimed subject matter also may be implemented in combination with other program modules. Generally, program modules include routines, programs, components, data structures, etc. that perform particular tasks and/or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the inventive methods may be practiced with other computer system configurations, including single-processor or multiprocessor computer systems, mini-computing devices, mainframe computers, as well as personal computers, hand-held computing devices (e.g., personal digital assistant (PDA), phone, watch . . . ), microprocessor-based or programmable consumer or industrial electronics, and the like. The illustrated aspects may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. However, some, if not all aspects of the subject matter can be practiced on stand-alone computers. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
   With reference to  FIG. 10 , an exemplary environment  1010  for implementing various aspects disclosed herein includes a computer  1012  (e.g., desktop, laptop, server, hand held, programmable consumer or industrial electronics . . . ). The computer  1012  includes a processing unit  1014 , a system memory  1016 , and a system bus  1018 . The system bus  1018  couples system components including, but not limited to, the system memory  1016  to the processing unit  1014 . The processing unit  1014  can be any of various available microprocessors. Dual microprocessors and other multiprocessor architectures also can be employed as the processing unit  1014 . 
   The system bus  1018  can be any of several types of bus structure(s) including the memory bus or memory controller, a peripheral bus or external bus, and/or a local bus using any variety of available bus architectures including, but not limited to, 11-bit bus, Industrial Standard Architecture (ISA), Micro-Channel Architecture (MSA), Extended ISA (EISA), Intelligent Drive Electronics (IDE), VESA Local Bus (VLB), Peripheral Component Interconnect (PCI), Universal Serial Bus (USB), Advanced Graphics Port (AGP), Personal Computer Memory Card International Association bus (PCMCIA), and Small Computer Systems Interface (SCSI). 
   The system memory  1016  includes volatile memory  1020  and nonvolatile memory  1022 . The basic input/output system (BIOS), containing the basic routines to transfer information between elements within the computer  1012 , such as during start-up, is stored in nonvolatile memory  1022 . By way of illustration, and not limitation, nonvolatile memory  1022  can include read only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable ROM (EEPROM), or flash memory. Volatile memory  1020  includes random access memory (RAM), which acts as external cache memory. By way of illustration and not limitation, RAM is available in many forms such as synchronous RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), Synchlink DRAM (SLDRAM), and direct Rambus RAM (DRRAM). 
   Computer  1012  also includes removable/non-removable, volatile/non-volatile computer storage media.  FIG. 10  illustrates, for example, disk storage  1024 . Disk storage  1024  includes, but is not limited to, devices like a magnetic disk drive, floppy disk drive, tape drive, Jaz drive, Zip drive, LS- 100  drive, flash memory card, or memory stick. In addition, disk storage  1024  can include storage media separately or in combination with other storage media including, but not limited to, an optical disk drive such as a compact disk ROM device (CD-ROM), CD recordable drive (CD-R Drive), CD rewritable drive (CD-RW Drive) or a digital versatile disk ROM drive (DVD-ROM). To facilitate connection of the disk storage devices  1024  to the system bus  1018 , a removable or non-removable interface is typically used such as interface  1026 . 
   It is to be appreciated that  FIG. 10  describes software that acts as an intermediary between users and the basic computer resources described in suitable operating environment  1010 . Such software includes an operating system  1028 . Operating system  1028 , which can be stored on disk storage  1024 , acts to control and allocate resources of the computer system  1012 . System applications  1030  take advantage of the management of resources by operating system  1028  through program modules  1032  and program data  1034  stored either in system memory  1016  or on disk storage  1024 . It is to be appreciated that the claimed subject matter can be implemented with various operating systems or combinations of operating systems. 
   A user enters commands or information into the computer  1012  through input device(s)  1036 . Input devices  1036  include, but are not limited to, a pointing device such as a mouse, trackball, stylus, touch pad, keyboard, microphone, joystick, game pad, satellite dish, scanner, TV tuner card, digital camera, digital video camera, web camera, and the like. These and other input devices connect to the processing unit  1014  through the system bus  1018  via interface port(s)  1038 . Interface port(s)  1038  include, for example, a serial port, a parallel port, a game port, and a universal serial bus (USB). Output device(s)  1040  use some of the same type of ports as input device(s)  1036 . Thus, for example, a USB port may be used to provide input to computer  1012  and to output information from computer  1012  to an output device  1040 . Output adapter  1042  is provided to illustrate that there are some output devices  1040  like displays (e.g., flat panel and CRT), speakers, and printers, among other output devices  1040  that require special adapters. The output adapters  1042  include, by way of illustration and not limitation, video and sound cards that provide a means of connection between the output device  1040  and the system bus  1018 . It should be noted that other devices and/or systems of devices provide both input and output capabilities such as remote computer(s)  1044 . 
   Computer  1012  can operate in a networked environment using logical connections to one or more remote computers, such as remote computer(s)  1044 . The remote computer(s)  1044  can be a personal computer, a server, a router, a network PC, a workstation, a microprocessor based appliance, a peer device or other common network node and the like, and typically includes many or all of the elements described relative to computer  1012 . For purposes of brevity, only a memory storage device  1046  is illustrated with remote computer(s)  1044 . Remote computer(s)  1044  is logically connected to computer  1012  through a network interface  1048  and then physically connected via communication connection(s)  1050 . Network interface  1048  encompasses communication networks such as local-area networks (LAN) and wide-area networks (WAN). LAN technologies include Fiber Distributed Data Interface (FDDI), Copper Distributed Data Interface (CDDI), Ethernet/IEEE 802.3, Token Ring/IEEE 802.5 and the like. WAN technologies include, but are not limited to, point-to-point links, circuit-switching networks like Integrated Services Digital Networks (ISDN) and variations thereon, packet switching networks, and Digital Subscriber Lines (DSL). 
   Communication connection(s)  1050  refers to the hardware/software employed to connect the network interface  1048  to the bus  1018 . While communication connection  1050  is shown for illustrative clarity inside computer  1012 , it can also be external to computer  1012 . The hardware/software necessary for connection to the network interface  1048  includes, for exemplary purposes only, internal and external technologies such as, modems including regular telephone grade modems, cable modems, power modems and DSL modems, ISDN adapters, and Ethernet cards or components. 
     FIG. 11  is a schematic block diagram of a sample-computing environment  1100  with which the claimed subject matter can interact. The system  1100  includes one or more client(s)  1110 . The client(s)  1110  can be hardware and/or software (e.g., threads, processes, computing devices). The system  1100  also includes one or more server(s)  1130 . Thus, system  1100  can correspond to a two-tier client server model or a multi-tier model (e.g., client, middle tier server, data server), amongst other models. The server(s)  1130  can also be hardware and/or software (e.g., threads, processes, computing devices). One possible communication between a client  1110  and a server  1130  may be in the form of a data packet adapted to be transmitted between two or more computer processes. The system  1100  includes a communication framework  1150  that can be employed to facilitate communications between the client(s)  1110  and the server(s)  1130 . The client(s)  1110  are operably connected to one or more client data store(s)  1160  that can be employed to store information local to the client(s)  1110 . Similarly, the server(s)  1130  are operably connected to one or more server data store(s)  1140  that can be employed to store information local to the servers  1130 . 
   What has been described above includes examples of aspects of the claimed subject matter. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the claimed subject matter, but one of ordinary skill in the art may recognize that many further combinations and permutations of the disclosed subject matter are possible. Accordingly, the disclosed subject matter is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the terms “includes,” “has” or “having” are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim.