封装包含两个参数的方法委托，没有返回值。

语法

public delegate void Action
     
      (    T1 arg1,    T2 arg2)
     

类型参数

in T1：委托封装方法的第一个参数类型，此类型参数逆变。

用法

可以使用Action<T1, T2>委托以参数形式传递方法，而不用自定义委托。封装的方法必须与此委托的方法签名一致。也就是说，封装的方法也要有两个参数，没有返回值。

下面显式声明了一个名为ConcatStrings的委托。然后，它将两个方法中的任意一个的引用分配给其委托实例。其中一个方法将两个字符串写入控制台；另一个将两个字符串写入文件。

using System;using System.IO;delegate void ConcatStrings(string string1, string string2);public class TestDelegate{   public static void Main()   {      string message1 = "The first line of a message.";      string message2 = "The second line of a message.";      ConcatStrings concat;      if (Environment.GetCommandLineArgs().Length > 1)         concat = WriteToFile;      else         concat = WriteToConsole;      concat(message1, message2);   }   private static void WriteToConsole(string string1, string string2)   {      Console.WriteLine("{0}\n{1}", string1, string2);               }   private static void WriteToFile(string string1, string string2)   {      StreamWriter writer = null;        try      {         writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);         writer.WriteLine("{0}\n{1}", string1, string2);      }      catch      {         Console.WriteLine("File write operation failed...");      }      finally      {         if (writer != null) writer.Close();      }         }}

下面以Action<T1, T2>委托简化上面的代码：

using System;using System.IO;public class TestAction2{   public static void Main()   {      string message1 = "The first line of a message.";      string message2 = "The second line of a message.";      Action
     
       concat;      if (Environment.GetCommandLineArgs().Length > 1)         concat = WriteToFile;      else         concat = WriteToConsole;      concat(message1, message2);   }   private static void WriteToConsole(string string1, string string2)   {      Console.WriteLine("{0}\n{1}", string1, string2);               }   private static void WriteToFile(string string1, string string2)   {      StreamWriter writer = null;        try      {         writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);         writer.WriteLine("{0}\n{1}", string1, string2);      }      catch      {         Console.WriteLine("File write operation failed...");      }      finally      {         if (writer != null) writer.Close();      }         }}
     

其实就是预先定义好的委托，不需要自定义对应参数的委托了。

还可以同匿名方法一起使用：

using System;using System.IO;public class TestAnonymousMethod{   public static void Main()   {      string message1 = "The first line of a message.";      string message2 = "The second line of a message.";      Action
     
       concat;      if (Environment.GetCommandLineArgs().Length > 1)         concat = delegate(string s1, string s2) { WriteToFile(s1, s2); };      else         concat = delegate(string s1, string s2) { WriteToConsole(s1, s2);} ;      concat(message1, message2);   }   private static void WriteToConsole(string string1, string string2)   {      Console.WriteLine("{0}\n{1}", string1, string2);               }   private static void WriteToFile(string string1, string string2)   {      StreamWriter writer = null;        try      {         writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);         writer.WriteLine("{0}\n{1}", string1, string2);      }      catch      {         Console.WriteLine("File write operation failed...");      }      finally      {         if (writer != null) writer.Close();      }         }}
     

也可以将匿名函数替换为lambda表达式：

using System;using System.IO;public class TestLambdaExpression{   public static void Main()   {      string message1 = "The first line of a message.";      string message2 = "The second line of a message.";      Action
     
       concat;      if (Environment.GetCommandLineArgs().Length > 1)         concat = (s1, s2) => WriteToFile(s1, s2);      else         concat = (s1, s2) => WriteToConsole(s1, s2);      concat(message1, message2);   }   private static void WriteToConsole(string string1, string string2)   {      Console.WriteLine("{0}\n{1}", string1, string2);               }   private static void WriteToFile(string string1, string string2)   {      StreamWriter writer = null;        try      {         writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);         writer.WriteLine("{0}\n{1}", string1, string2);      }      catch      {         Console.WriteLine("File write operation failed...");      }      finally      {         if (writer != null) writer.Close();      }         }}
     

后两者都没有起到简化代码的作用。