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Action<T1, T2> Delegate

Updated: October 2010

Encapsulates a method that has two parameters and does not return a value.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

[TypeForwardedFromAttribute("System.Core, Version=2.0.5.0, Culture=Neutral, PublicKeyToken=7cec85d7bea7798e")]
public delegate void Action<in T1, in T2>(
	T1 arg1,
	T2 arg2
)

Type Parameters

in T1

The type of the first parameter of the method that this delegate encapsulates.

This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see 2678dc63-c7f9-4590-9ddc-0a4df684d42e.

in T2

The type of the second parameter of the method that this delegate encapsulates.

Parameters

arg1
Type: T1
The first parameter of the method that this delegate encapsulates.
arg2
Type: T2
The second parameter of the method that this delegate encapsulates.

You can use the Action<T1, T2> delegate to pass a method as a parameter without explicitly declaring a custom delegate. The encapsulated method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have two parameters that are both passed to it by value, and it must not return a value. (In C#, the method must return void. In Visual Basic, it must be defined by the SubEnd Sub construct. It can also be a method that returns a value that is ignored.) Typically, such a method is used to perform an operation.

NoteNote:

To reference a method that has two parameters and returns a value, use the generic Func<T1, T2, TResult> delegate instead.

When you use the Action<T1, T2> delegate, you do not have to explicitly define a delegate that encapsulates a method with two parameters. For example, the following code explicitly declares a delegate named ConcatStrings. It then assigns a reference to either of two methods to its delegate instance. One method writes two strings to the console; the second writes two strings to a message box.


using System;

delegate void ConcatStrings(string string1, string string2);

public class Example
{
   private static System.Windows.Controls.TextBlock outputBlock;

   public static void Demo(System.Windows.Controls.TextBlock outBlock)
   {
      outputBlock = outBlock;

      string message1 = "The first line of a message.";
      string message2 = "The second line of a message.";
      ConcatStrings concat;

      // Generate a random number and use it to determine which method is
      // assigned to the delegate;
      Random rnd = new Random();
      if (rnd.NextDouble() <= .5) 
         concat = WriteToWindow;
      else
         concat = WriteToConsole;

      concat(message1, message2);
   }

   private static void WriteToConsole(string string1, string string2)
   {
      outputBlock.Text += String.Format("{0}\n{1}", string1, string2) + "\n";
   }

   private static void WriteToWindow(string string1, string string2)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(string.Format("{0}\n{1}", 
                                                   string1, string2));
   }
}


The following example simplifies this code by instantiating the Action<T1, T2> delegate rather than explicitly defining a new delegate and assigning a named method to it.


using System;

public class Example
{
   private static System.Windows.Controls.TextBlock outputBlock;

   public static void Demo(System.Windows.Controls.TextBlock outBlock)
   {
      outputBlock = outBlock;
      string message1 = "The first line of a message.";
      string message2 = "The second line of a message.";
      Action<string, string> concat;

      // Generate a random number and use it to determine which method is
      // assigned to the delegate;
      Random rnd = new Random();
      if (rnd.NextDouble() <= .5) 
         concat = WriteToWindow;
      else
         concat = WriteToConsole;

      concat(message1, message2);
   }

   private static void WriteToConsole(string string1, string string2)
   {
      outputBlock.Text += String.Format("{0}\n{1}\n", string1, string2);
   }

   private static void WriteToWindow(string string1, string string2)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(String.Format("{0}\n{1}\n", 
                                                   string1, string2));
   }
}


You can also use the Action<T1, T2> delegate with anonymous methods in C#, as the following example illustrates.


using System;

public class TestAnonymousMethod
{
   private static System.Windows.Controls.TextBlock outputBlock;

   public static void Demo(System.Windows.Controls.TextBlock outBlock)
   {
      outputBlock = outBlock;

      string message1 = "The first line of a message.";
      string message2 = "The second line of a message.";
      Action<string, string> concat;

      // Generate a random number and use it to determine which method is
      // assigned to the delegate;
      Random rnd = new Random();
      if (rnd.NextDouble() <= .5) 
         concat = delegate(string s1, string s2) { WriteToWindow(s1, s2); };
      else
         concat = delegate(string s1, string s2) { WriteToConsole(s1, s2); };

      concat(message1, message2);
   }

   private static void WriteToConsole(string string1, string string2)
   {
      outputBlock.Text += String.Format("{0}\n{1}", string1, string2) + "\n";
   }

   private static void WriteToWindow(string string1, string string2)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(string.Format("{0}\n{1}", 
                                                   string1, string2));
   }
}


You can also assign a lambda expression to an Action<T1, T2> delegate instance, as the following example illustrates.


using System;

public class Example
{
   private static System.Windows.Controls.TextBlock outputBlock;

   public static void Demo(System.Windows.Controls.TextBlock outBlock)
   {
      outputBlock = outBlock;

      string message1 = "The first line of a message.";
      string message2 = "The second line of a message.";
      Action<string, string> concat;

      // Generate a random number and use it to determine which method is
      // assigned to the delegate;
      Random rnd = new Random();
      if (rnd.NextDouble() <= .5) 
         concat = (s1, s2) => WriteToWindow(s1, s2);
      else
         concat = (s1, s2) => WriteToConsole(s1, s2);

      concat(message1, message2);
   }

   private static void WriteToConsole(string string1, string string2)
   {
      outputBlock.Text += String.Format("{0}\n{1}", string1, string2) + "\n";
   }

   private static void WriteToWindow(string string1, string string2)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(string.Format("{0}\n{1}", 
                                                   string1, string2));
   }
}


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Date

History

Reason

October 2010

Modified Visual Basic lambda expression to use Sub keyword.

Customer feedback.

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