Action<T1, T2>委托

封裝包含兩個參數(shù)的方法委托，沒有返回值。

語法

public delegate void Action<in T1, in T2>(    T1 arg1,    T2 arg2)

類型參數(shù)

in T1：委托封裝方法的第一個參數(shù)類型，此類型參數(shù)逆變。

用法

可以使用Action<T1, T2>委托以參數(shù)形式傳遞方法，而不用自定義委托。封裝的方法必須與此委托的方法簽名一致。也就是說，封裝的方法也要有兩個參數(shù)，沒有返回值。

下面顯式聲明了一個名為ConcatStrings的委托。然后，它將兩個方法中的任意一個的引用分配給其委托實例。其中一個方法將兩個字符串寫入控制臺；另一個將兩個字符串寫入文件。

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>委托簡化上面的代碼：

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<string, string> 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();      }         }}

其實就是預(yù)先定義好的委托，不需要自定義對應(yīng)參數(shù)的委托了。

還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<string, string> 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();      }         }}

也可以將匿名函數(shù)替換為lambda表達式：

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<string, string> 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();      }         }}

后兩者都沒有起到簡化代碼的作用。