C# doesn't support multiple inheritances but it is been achieved using interfaces. A class in C# has the option to implement one or more interface(S). There is only one challenge when class implements multiple interfaces is that they may include methods with same name and signature as existing class members or other interfaces' members.

Explicit interface implementation gives the ability to hide the details of an interface and it is only available by casting to the interface type. Explicit interface implementation can be used to disambiguate class members and interface members otherwise they will conflict.

Lets take an example, If a class implements two interfaces that contain a member with the same signature, then implementing that member on the class will cause both interfaces to use that member as their implementation.

interface IInterfaceA

{

    void Paint();

}

interface IInterfaceB

{

   void Paint();

}

class SampleClassA : IInterfaceA, IInterfaceB

{

    // Both IInterfaceA.Paint and IInterfaceA.Paint call this method.

    public void Paint()

    {

    }

}

The class member IInterfaceA.Paint is only available through the IInterfaceA interface, and IInterfaceB.Paint is only available through IInterfaceB. Both method implementations are separate, and neither is available directly on the class. For example:

SampleClass obj = new SampleClass();
//obj.Paint();  // Compiler error.

IInterfaceA c = (IInterfaceA)obj;
c.Paint();  // Calls IInterfaceA.Paint on SampleClass.

IInterfaceB s = (IInterfaceB)obj;
s.Paint(); // Calls IInterfaceB.Paint on SampleClass.

It is possible to implement an interface member explicitly. Create a class member that is only called through the interface, and is specific to that interface. This can be achieved by naming the class member with the name of the interface and a period. For example:

public class SampleClass : IInterfaceA, IInterfaceB

{

   void IInterfaceA.Paint()

   {

        System.Console.WriteLine("IInterfaceA.Paint");

   }

   void IInterfaceB.Paint()

   {

        System.Console.WriteLine("IInterfaceB.Paint");

   }

}

An interface contains only the signatures of methods,  events, indexers or properties. When a class or struct implements the interface then that class/struct must implement the members of the interface that are specified in the interface definition.

An interface cannot contain fields and members are automatically public in interface as well and an interface can inherit from one or more base interfaces. When a base type list contains a base class and interfaces, the base class must come first in the list.

A class that implements an interface can explicitly implement members of that interface. An explicitly implemented member cannot be accessed through a class instance, but only through an instance of the interface.

Interfaces can inherit other interfaces. It is possible for a class to inherit an interface multiple times, through base classes or interfaces it inherits. In this case, the class can only implement the interface one time, if it is declared as part of the new class. If the inherited interface is not declared as part of the new class, its implementation is provided by the base class that declared it. It is possible for a base class to implement interface members using virtual members; in that case, the class inheriting the interface can change the interface behavior by overriding the virtual members.

it was necessary to incorporate some other method so that the class can inherit the behavior of more than one class, avoiding the problem of name ambiguity that is found in C++. With name ambiguity, the object of a class does not know which method to call if the two base classes of that class object contain the same named method.

Interfaces Overview

An interface has the following properties:

•       An interface is like an abstract base class: any non-abstract type inheriting the interface must implement all its members.
•       An interface cannot be instantiated directly.
•       Interfaces can contain events, indexers, methods and properties.
•       Interfaces contain no implementation of methods.
•       Classes and structs can inherit from more than one interface.
•       An interface can itself inherit from multiple interfaces.

Both delegates and interfaces allow a class designer to separate type declarations and implementation. A given interface can be inherited and implemented by any class or struct; a delegate can created for a method on any class, as long as the method fits the method signature for the delegate. An interface reference or a delegate can be used by an object with no knowledge of the class that implements the interface or delegate method. Given these similarities, when should a class designer use a delegate and when should they use an interface?

Use a delegate when:

• An eventing design pattern is used.

• It is desirable to encapsulate a static method.

• The caller has no need access other properties, methods, or interfaces on the object implementing the method.

• Easy composition is desired.

• A class may need more than one implementation of the method.

Use an interface when:

• There are a group of related methods that may be called.

• A class only needs one implementation of the method.

• The class using the interface will want to cast that interface to other interface or class types.

• The method being implemented is linked to the type or identity of the class: for example, comparison methods.

One good example of using a single-method interface instead of a delegate is IComparable or IComparable. IComparable declares the CompareTo method, which returns an integer specifying a less than, equal to, or greater than relationship between two objects of the same type. IComparable can be used as the basis of a sort algorithm, and while using a delegate comparison method as the basis of a sort algorithm would be valid, it is not ideal. Because the ability to compare belongs to the class, and the comparison algorithm doesn’t change at run-time, a single-method interface is ideal.

Extracted from Microsoft Technet articles.