封装包含两个参数的方法委托，没有返回值。
语法
public delegate void Action<in T1, in T2>(
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<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<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<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();
}
}
}
后两者都没有起到简化代码的作用。