扩展方法与接口相结合

使用带接口的扩展方法非常方便，因为实现可以存储在类之外，并且向类添加一些功能所需要的是用接口来装饰类。

public interface IInterface
{
   string Do()
}

public static class ExtensionMethods{
    public static string DoWith(this IInterface obj){
      //does something with IInterface instance
    }
}

public class Classy : IInterface
{
   // this is a wrapper method; you could also call DoWith() on a Classy instance directly,
   // provided you import the namespace containing the extension method
   public Do(){
       return this.DoWith();
   }
}

使用像：

 var classy = new Classy();
 classy.Do(); // will call the extension
 classy.DoWith(); // Classy implements IInterface so it can also be called this way