C#: Interfaces

Interfaces are the contracts in the outside world.

What it means?

Take an example, let us assume, there is a standard for all mobile phones, as per the standard, below are the basic functionality for the mobile phones to provide.

Vendor1 provides the functionality for the mobile in language JAVA

Vendor2 provides the functionality for the mobile in language C++

Vendor3 provides the functionality for the mobile in language JAVA

Vendor4 provides the functionality for the mobile in language C#

That mean all the vendors providing the same functionality, but the way they are providing is different.

One more example is IBM JAVA, Oracle JAVA implements Java specification, but the way they implemented the features is different.

Interfaces in C#

In C#, interfaces are reference types contain only method signatures.

Interfaces doesn't contain method bodies, these are implemented by classes that extend (or) inherit these interfaces

Defining an Interface

   interface InterfaceName{

      // method signatures

   }

Note

All the methods in the interface are public.

Implementing an Interface

A class that implements the interface provides the method body for the method signatures in the interface.

Syntax

class ClassName : Interface1, Interface2, ...Interface N{

        //implements the interfaces

 }

Program.cs

using System;

interface Circle
{
    double getArea(int r);
}

class MyCircle : Circle
{
    private static double PI = 3.1428;

    public double getArea(int r)
    {
        return (PI * r * r);
    }
}

class Program
{
  
    static void Main(string[] args)
    {
        MyCircle circle = new MyCircle();

        double area = circle.getArea(10);

        Console.WriteLine("Area of the Circle is {0}", area);
    }
}

Output

Area of the Circle is 314.28

Some Points to Remember

1.If a class implements an interface, then it must implement all the methods in the interface, otherwise, the class must declared as abstract.

2. By default all the methods in the interface are public, while implementing the interface, class must provide the public access specifier for the methods it implementing.

Interface as Reference Types

Interfaces can be used as Reference types. If you define a reference variable whose type is an interface, any object you assign to it must be an instance of a class that implements the interface.

Example

Circle circle = new MyCircle();

Program.cs

using System;

interface Circle
{
    double getArea(int r);
}

class MyCircle : Circle
{
    private static double PI = 3.1428;

    public double getArea(int r)
    {
        return (PI * r * r);
    }
}

class Program
{
  
    static void Main(string[] args)
    {
        Circle circle = new MyCircle();

        double area = circle.getArea(10);

        Console.WriteLine("Area of the Circle is {0}", area);
    }
}

Output

Area of the Circle is 314.28

By assigning an object to the interface type, the reference variable “circle1” call only the methods declared in the interface. Trying to call the methods which are not declared in the interface cause the compiler error.

Program.cs

using System;

interface Circle
{
    double getArea(int r);
}

class MyCircle : Circle
{
    private static double PI = 3.1428;

    public double getArea(int r)
    {
        return (PI * r * r);
    }

    public double getPerimeter(int r)
    {
        return (PI * 2 * r);
    }

}

class Program
{
  
    static void Main(string[] args)
    {
        Circle circle = new MyCircle();

        circle.getPerimeter(10);
    }
}

Above program, tries to call the getPerimeter(), which is not declared in the interface Circle. Your program don’t compile.

Class can implement more than one interface

In Java, a class can implement more than one interface.

Syntax

    class ClassName : Interface1, Interface2{

    }

Program.cs

using System;

interface Area
{
    double getArea(double r);
}

interface Perimeter
{
    double getPerimeter(double r);
}

class MyCircle : Area, Perimeter{
   double PI = 3.1428;

    public double getArea(double r)
    {
        return (PI * r * r);
    }

    public double getPerimeter(double r)
    {
        return (PI * 2 * r);
    }
}

class Program
{
  
    static void Main(string[] args)
    {
        MyCircle circle = new MyCircle();

        double perimeter = circle.getPerimeter(10);
        double area = circle.getArea(10);

        Console.WriteLine("Perimeter :  {0}", perimeter);
        Console.WriteLine("Area :  {0}", area);
    }
}

Output

Perimeter :  62.856

Area :  314.28

Interface extends other interface

In C#, one interface can extends other interface.

Syntax

   interface Interface3 : Interface1, Interface2{

   }

Class that implementing the Interface3 must provide implementation for all the methods in Interface1, Interface2 and Interface3.

Program.cs

using System;

interface Interface1
{
    int sum(int a, int b);
}

interface Interface2 : Interface1
{
    int sum(int a, int b, int c);
}

class Sum : Interface2
{
    public int sum(int a, int b, int c)
    {
        return (a + b + c);
    }

    public int sum(int a, int b)
    {
        return (a + b);
    }
}

class Program
{
  
    static void Main(string[] args)
    {
        Sum sumCalc = new Sum();

        int result1 = sumCalc.sum(10, 20);
        int result2 = sumCalc.sum(10, 20, 30);

        Console.WriteLine("result1 : {0}", result1);
        Console.WriteLine("result2 : {0}", result2);
    }
}

Output

result1 : 30

result2 : 60

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