The extension method.

          When you define extension methods, the first restriction is that they must be defined within a static class (see Chapter 5) and, therefore, each extension method must be declared with the static keyword.

          当你定义扩展方法时候，第一个约束他们必须定义在静态类里面，此外，每个扩展方法必须使用static关键字来声明。

         The second point is that all extension methods are marked as such by using the this keyword as a modifier on the first (and only the first) parameter of the method in question.