C# method implementation with dot notation

Question

Reading an article I came across the following C# syntax in method name.

private class sortYearAscendingHelper : IComparer
{
   int IComparer.Compare(object a, object b)
   {
       ...
   }
}

I understand Compare method is method of IComparer interface, but coming from C++ I am not certain what this syntax means. If Compare is part of interface, I would expect to mention that only like int Compare(...). Why we have to specify class?

Answer 1

That is an explicit interface implementation You use it when you derive from multiple interfaces that contain similar (same signature) functions but need different implementations for each interface.

More information can be found on MSDN.

(Sample from linked page):

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.