An Interface is similar to an Abstract class and it also cannot be instantiated.

•      Cannot be instantiated i.e. object cannot be created using the new keyword.

•      Can contain only declaration of Members and not definition.

•      Members are simply declared and are defined in the classes deriving the Interface.

•      Members are defined without using the override keyword.

•      Cannot contain Access modifiers i.e. Members cannot be public, private, etc.

•      The derived member can only be public.

•      Does support Multiple Inheritance.
Example:
public interface A
{
    void Fun1();
    void Fun2();
}
public class B : A
{
    public void Fun1()
    {
    }
    //Error: 'B' does not implement interface member 'A.Fun2()'
    //'B.Fun2()' cannot implement an interface member because it is not public
    private void Fun2()
    {
    }
}
public class C
{
    public static void Fun3()
    {
        //Compiler Error: Cannot create an instance of the abstract class or interface 'A'
        A a = new A();
 
        //Valid
        B b = new B();
        B b = new B();
        b.Fun1();
 
        //Invalid
        b.Fun2();
    }
}