Explicit Interface Implementation (C# Programming Guide) 

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. For example:

interface IControl
{
    void Paint();
}
interface ISurface
{
    void Paint();
}
class SampleClass : IControl, ISurface
{
    // Both ISurface.Paint and IControl.Paint call this method.
    public void Paint()
    {
    }
}

If the two interface members do not perform the same function, however, this can lead to an incorrect implementation of one or both of the interfaces. It is possible to implement an interface member explicitly—creating a class member that is only called through the interface, and is specific to that interface. This is accomplished by naming the class member with the name of the interface and a period. For example:

public class SampleClass : IControl, ISurface
{
    void IControl.Paint()
    {
        System.Console.WriteLine("IControl.Paint");
    }
    void ISurface.Paint()
    {
        System.Console.WriteLine("ISurface.Paint");
    }
}

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

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

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

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

Explicit implementation is also used to resolve cases where two interfaces each declare different members of the same name such as a property and a method:

interface ILeft
{
    int P { get;}
}
interface IRight
{
    int P();
}

To implement both interfaces, a class has to use explicit implementation either for the property P, or the method P, or both, to avoid a compiler error. For example:

class Middle : ILeft, IRight
{
    public int P() { return 0; }
    int ILeft.P { get { return 0; } }
}

See Also

Reference

Objects, Classes and Structs (C# Programming Guide)
Interfaces (C# Programming Guide)
Inheritance (C# Programming Guide)

Concepts

C# Programming Guide