C# | How to use Interface References - GeeksforGeeks

11 Jun, 2019
In C#, you are allowed to create a reference variable of an interface type or in other words, you are allowed to create an interface reference variable. Such kind of variable can refer to any object that implements its interface. An interface reference variable only knows that methods which are declared by its interface declaration. It does not allow accessing any other variables or methods that might be supported by the objects. This concept is similar when you use a parent class reference to access a child class object.
Below are the examples to illustrate the concept of Interface References:
Example 1:
// C# program to illustrate the // concept of Interface Referencesusing System;  // interface declarationpublic interface Race {          // declaration of abstract methods of    // interface that will be implemented     // by the class which inherits the interface    void Speed(int s);    void Distance(int d);}  // class implementing interfacepublic class Person1 : Race {          int sp1, di1;          // abstract method of     // Race interface    public void Speed(int p1s) {                  sp1 = p1s;        Console.WriteLine("Speed Method implemented by Person1");              }          // abstract method of     // Race interface    public void Distance(int p1d) {                  di1 = p1d;        Console.WriteLine("Distance Method implemented by Person1");              }          // method of class Person1    public void display1() {                  Console.WriteLine("The Speed of 1st person is: "+sp1);        Console.WriteLine("The distance covered by 1st person is: "+di1);              }      }  // class implementing interfacepublic class Person2 : Race {          int sp2, di2;          // abstract method of     // Race interface    public void Speed(int p2s) {                  sp2 = p2s;        Console.WriteLine("Speed Method implemented by Person2");              }          // abstract method of     // Race interface    public void Distance(int p2d) {                  di2 = p2d;        Console.WriteLine("Distance Method implemented by Person2");              }          // method of class Person2    public void display2() {                  Console.WriteLine("The Speed of 2nd person is: "+sp2);        Console.WriteLine("The distance covered by 2nd person is: "+di2);              }      }  // Driver Classpublic class GFG {          // Main method    public static void Main(String []args) {                  // creating an instance of Person1 class        Person1 obj1 = new Person1();                  // creating an instance of Person2 class        Person2 obj2 = new Person2();                  // creating an Reference         // of interface Race        Race r;                  // ----- For Person1 Class ----------                  // assigning Person1 object 'obj1'        // to interface Reference 'r'        r = obj1;                  // Now you can access the abstract method        // of Race interface which are implemented        // by class Person1        r.Speed(10);        r.Distance(50);          // if you will try to call display1()         // method using 'r' it will give error        //r.display1();                  // calling the display1()         // method of Person1 Class        obj1.display1();                  // ----- For Person2 Class ----------                  // assigning Person2 object 'obj2'        // to interface Reference 'r'        r = obj2;                  // Now you can access the abstract method        // of Race interface which are implemented        // by class Person2        r.Speed(15);        r.Distance(45);          // if you will try to call display2()         // method using 'r' it will give error        //r.display2();                  // calling the display1()         // method of Person1 Class        obj2.display2();              }      }
Speed Method implemented by Person1
Distance Method implemented by Person1
The Speed of 1st person is: 10
The distance covered by 1st person is: 50
Speed Method implemented by Person2
Distance Method implemented by Person2
The Speed of 2nd person is: 15
The distance covered by 2nd person is: 45

Explanation: In above example, we have an interface named Race an two classes Person1 and Person2 which are implementing the methods of the interface. The Person1 class has its own method named display1() and similar Person2 class its own method display2() which cannot be called by using interface reference. In order to call the methods using interface reference(here r is interface reference), you have to assign to class object to it. Like if you are assigning Person1’s object obj1 to r i.e. r = obj1; then you call the Speed() and Distance() methods that are implemented by the Person1 class. In order to call display1() method, you must have to use obj1. Similarly using r = obj2; we are calling methods of Person2 class.
Example 2:
// C# program to illustrate the // concept of Interface Referencesusing System;  // interface declarationinterface Vehicle {      // all are the abstract methods.    void changeGear(int a);    void speedUp(int a);    void applyBrakes(int a);    void printStates();}  // class implements interfaceclass Bicycle : Vehicle {      int speed;    int gear;      // to change gear    public void changeGear(int newGear)    {          gear = newGear;    }      // to increase speed    public void speedUp(int increment)    {          speed = speed + increment;    }      // to decrease speed    public void applyBrakes(int decrement)    {          speed = speed - decrement;    }      public void printStates()    {        Console.WriteLine("speed: " + speed + " gear: " + gear);    }}  // Driver Classclass GFG {      // Main Method    public static void Main(String[] args)    {          // creating an instance of Bicycle        Bicycle bicycle = new Bicycle();          // Creating interface references        Vehicle obj;          // assigning Bicycle object 'bicycle'        // to interface Reference 'obj'        obj = bicycle;          // calling the abstract methods         // implemented by class Bicycle        obj.changeGear(4);        obj.speedUp(5);        obj.applyBrakes(2);          Console.WriteLine("Bicycle Present State:");          // calling the method of class Bicycle         obj.printStates();    }}
Bicycle Present State:
speed: 3 gear: 4

Explanation: In the above example, Vehicle is an interface and Bicycle class implements this interface. Here obj is declared to be a reference to Vehicle interface in the Main() method. Now this obj is used to refer the object bicycle of the Bicycle class.
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