Extension Methods - MSDN
An extension method with the same name and signature as an
interface or class method will never be called. At compile time,
extension methods always have lower priority than instance methods defined in the type itself.
You can call the extension method as regular static method of a class.
ExtenstionTest.MethodA(a);
From the MSDN
In other words, if a type has a method named Process(int i), and you
have an extension method with the same signature, the compiler will
always bind to the instance method. When the compiler encounters a
method invocation, it first looks for a match in the type's instance
methods. If no match is found, it will search for any extension
methods that are defined for the type, and bind to the first extension
method that it finds. The following example demonstrates how the
compiler determines which extension method or instance method to bind
to.
