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Action<T> Delegate

Updated: October 2010

Encapsulates a method that takes a single parameter and does not return a value.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
public delegate void Action<in T>(
	T obj
)

Type Parameters

in T

The type of the 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.

Parameters

obj
Type: T
The parameter of the method that this delegate encapsulates.

You can use the Action<T> 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 one parameter that is 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 one parameter and returns a value, use the generic Func<T, TResult> delegate instead.

When you use the Action<T> delegate, you do not have to explicitly define a delegate that encapsulates a method with a single parameter. For example, the following code explicitly declares a delegate named DisplayMessage and assigns a reference to either the WriteLine method or the ShowWindowsMessage method to its delegate instance.


using System;

delegate void DisplayMessage(string message);

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

   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Example.outputBlock = outputBlock;
      DisplayMessage messageTarget; 

      Random rnd = new Random();
      if (rnd.NextDouble() <= .5)
         messageTarget = ShowAlertMessage;
      else
         messageTarget = ShowBrowserMessage;

      messageTarget("Hello, World!");   
   }      

   private static void ShowAlertMessage(string message)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(message);      
   }

   private static void ShowBrowserMessage(string message)
   {
      outputBlock.Text += message + "\n";
   }
}


The following example simplifies this code by instantiating the Action<T> 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 outputBlock)
   {
      Example.outputBlock = outputBlock;
      Action<string> messageTarget; 

      Random rnd = new Random();
      if (rnd.NextDouble() <= .5)
         messageTarget = ShowAlertMessage;
      else
         messageTarget = ShowBrowserMessage;

      messageTarget("Hello, World!");   
   }      

   private static void ShowAlertMessage(string message)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(message);      
   }

   private static void ShowBrowserMessage(string message)
   {
      outputBlock.Text += message + "\n";
   }
}


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


using System;

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

   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Example.outputBlock = outputBlock;
      Action<string> messageTarget; 

      Random rnd = new Random();
      if (rnd.NextDouble() <= .5)
         messageTarget = delegate(string s) { ShowAlertMessage(s); };
      else
         messageTarget = delegate(string s) { ShowBrowserMessage(s); };

      messageTarget("Hello, World!");
   }

   private static void ShowAlertMessage(string message)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(message);      
   }

   private static void ShowBrowserMessage(string message)
   {
      outputBlock.Text += message + "\n";
   }
}


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


using System;

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

   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Example.outputBlock = outputBlock;
      Action<string> messageTarget; 

      Random rnd = new Random();
      if (rnd.NextDouble() <= .5)
         messageTarget = s => ShowAlertMessage(s); 
      else
         messageTarget = s => ShowBrowserMessage(s);

      messageTarget("Hello, World!");
   }

   private static void ShowAlertMessage(string message)
   {
      System.Windows.Browser.HtmlPage.Window.Alert(message);      
   }
   private static void ShowBrowserMessage(string message)
   {
      outputBlock.Text += message + "\n";
   }
}


The ForEach and ForEach<T> methods each take an Action<T> delegate as a parameter. The method encapsulated by the delegate allows you to perform an action on each element in the array or list. The example uses the ForEach method to provide an illustration.

The following example demonstrates the use of the Action<T> delegate to print the contents of a List<T> object. In this example, the Print method is used to display the contents of the list to the console. In addition, the C# example also demonstrates the use of anonymous methods to display the contents to the console. Note that the example does not explicitly declare an Action<T> variable. Instead, it passes a reference to a method that takes a single parameter and that does not return a value to the List<T>.ForEach method, whose single parameter is an Action<T> delegate. Similarly, in the C# example, an Action<T> delegate is not explicitly instantiated because the signature of the anonymous method matches the signature of the Action<T> delegate that is expected by the List<T>.ForEach method.


using System;
using System.Collections.Generic;

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

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

      Example.outputBlock = outputBlock;
      List<String> names = new List<String>();
      names.Add("Bruce");
      names.Add("Alfred");
      names.Add("Tim");
      names.Add("Richard");

      // Display the contents of the list using the Print method.
      names.ForEach(Print);

      // The following demonstrates the anonymous method feature of C#
      // to display the contents of the list.
      names.ForEach(delegate(String name)
      {
         outputBlock.Text += name + "\n";
      });
   }

   private static void Print(string s)
   {
      outputBlock.Text += s + "\n";
   }
}
/* This code will produce output similar to the following:
 * Bruce
 * Alfred
 * Tim
 * Richard
 * Bruce
 * Alfred
 * Tim
 * Richard
 */


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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