Interface

To rectify the drawback of multiple inheritance, the generator  of C# have introduced a new concept called interfaces. Java programmers may be well aware of this concept. All interfaces should be declared with the keyword interface. You can implement any number of interfaces in a single derived class, but you should provide signatures to all method definitions of the corresponding interfaces.
To illustrate, Example 1 shows how to declare interfaces and implement them in a class:

Example 1

using System;
namespace InterfaceExample
{
interface InterAritical
{
void Show();
}

class ClsAritical : InterAritical
{
public void Show()
{
Console.WriteLine("Show() method Implemented");
}

public static void Main(string[] args)
{
ClsAritical clsAritical = new ClsAritical();
clsAritical.Show();
}
}
}
The output of this example is
Show() method Implemented

Combining Interfaces:
Two or more interfaces can be combined into a single interface and implemented in a class, as shown in Example 2:
Example 2
using System;
namespace InterfaceExample
{
interface InterAritical
{
void Show();
}

interface InterAritical1
{
void Display();
}

interface CombineInterAritical : InterAritical, InterAritical1
{
//Above interfaces combined
}

class ClsAritical : CombineInterAritical
{
public void Show()
{
Console.WriteLine("Show() method Implemented");
}

public void Display()
{
Console.WriteLine("Display() method Implemented");
}

public static void Main(string[] args)
{
ClsAritical clsAritical = new ClsAritical();
clsAritical.Show();
clsAritical.Display();
}
}
}
The output of the example
Show() method Implemented
Display() method Implemented

is and as oprator in interface:

You easily can determine whether a particular interface is implemented in a class by using is and as operators. The is operator enables you to check whether one type or class is compatible with another type or class; it returns a Boolean value. Example 3 illustrates the usage of the is operator.

Example 3
using System;
namespace InterfaceExample
{
interface InterAritical
{
bool Show();
}

interface InterAritical1
{
bool Display();
}
class ClsAritical : InterAritical
{
public bool Show()
{
Console.WriteLine("Show() method Implemented");
return true;
}
public static void Main(string[] args)
{
ClsAritical clsAritical = new ClsAritical();
clsAritical.Show();
if (clsAritical is InterAritical1)
{
InterAritical1 clsRefrence = (InterAritical1)clsAritical;
bool ok = clsRefrence.Display();
Console.WriteLine("Method Implemented");
}
else
{
Console.WriteLine("Method not implemented");
}
}
}
}
The output of the example is.
Show() method Implemented
Method not implemented