擴充套件方法與介面相結合

使用帶介面的擴充套件方法非常方便，因為實現可以儲存在類之外，並且向類新增一些功能所需要的是用介面來裝飾類。

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