Emanuele's blog

Quale sarà l'output?

namespace Extensions
{
  using System;
  using ExtensionMethodsDemo1;

     // Define extension methods for any type that implements IMyInterface.
     public static class Extension
     {
        public static void MethodA(this IMyInterface myInterface, int i)
        {
            Console.WriteLine("Extension.MethodA(this IMyInterface myInterface, int i)");
        }

        public static void MethodA(this IMyInterface myInterface, string s) 
        {
            Console.WriteLine("Extension.MethodA(this IMyInterface myInterface, string s)");
        }

        public static void MethodB(this IMyInterface myInterface) 
        {
            Console.WriteLine("Extension.MethodB(this IMyInterface myInterface)");
        }
    }
}
namespace ExtensionMethodsDemo1
{
    using System;
    using Extensions;

    public interface IMyInterface
    {
        void MethodB();
    }

    class A : IMyInterface 
    {
        public void MethodB(){Console.WriteLine("A.MethodB()");}
    } 

    class B : IMyInterface
    {
        public void MethodB() { Console.WriteLine("B.MethodB()"); }
        public void MethodA(int i) { Console.WriteLine("B.MethodA(int i)"); }
    }

    class C : IMyInterface
    {
        public void MethodB() { Console.WriteLine("C.MethodB()"); }
        public void MethodA(object obj) { Console.WriteLine("C.MethodA(object obj)"); }
    }

    class ExtMethodDemo
    {
        static void Main(string[] args)
        {
            A a = new A();
            B b = new B();
            C c = new C();
            TestMethodBinding(a,b,c);
        }

        static void TestMethodBinding(A a, B b, C c)
        {

	    //copy from here...
            a.MethodA(1);        //write here     
            a.MethodA("hello");  //write here
            a.MethodB();         //write here         

            b.MethodA(1);        //write here    
            b.MethodB();         //write here       
            b.MethodA("hello");  //write here   

            c.MethodA(1);        //write here  
            c.MethodA("hello");  //write here 
            c.MethodB();         //write here 

            //...to here 
        }
    }
}

[Fonte: MSDN]

Gli extension methods permettono a metodi statici di essere richiamati usando la sintassi dei metodi di istanza. Questi metodi devono avere almeno un parametro che rappresenta l'istanza del metodo su cui operare. In C# su questo parametro si utilizza la parola chiave this.

public static class StringExtensions{

    public static bool IsPalindrome(this string s){

       ...

    }

}

Questo metodo viene richiamato nel seguente modo:

if("hello world".IsPalindrome()){

  ....

}

La classe che definisce l'extension method viene definita "sponsor" e deve essere statica. Ovviamente va importato il namespace della classe sponsor.

Vanno usati il minimo indispensabile ;)

[Fonte: Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries (2nd Edition) - K. Cwalina, B. Abrams - Addison Wesley - 2009]