From MSDN
It is a compile-time error for an explicit interface member
implementation to include access modifiers, and it is a compile-time
error to include the modifiers abstract, virtual, override, or static.
Explicit
interface member implementations have different accessibility
characteristics than other members. Because explicit interface member
implementations are never accessible through their fully qualified name
in a method invocation or a property access, they are in a sense
private. However, since they can be accessed through an interface
instance, they are in a sense also public.
Refer : http://msdn.microsoft.com/en-us/library/aa664591%28v=vs.71%29.aspx