Action(Of 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)

'Declaration
Public Delegate Sub Action(Of In T) ( _
	obj As T _
)

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(Of 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(Of T, TResult) delegate instead.

When you use the Action(Of 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.


Delegate Sub DisplayMessage(message As String) 

Module Example
   Private outputBlock As System.Windows.Controls.TextBlock

   Public Sub Demo(outputBlock As System.Windows.Controls.TextBlock)
      Example.outputBlock = outputBlock 
      Dim messageTarget As DisplayMessage 

      Dim rnd As New Random()
      If rnd.NextDouble() <= .5
         messageTarget = AddressOf ShowAlertMessage
      Else
         messageTarget = AddressOf ShowBrowserMessage
      End If
      messageTarget("Hello, World!")
   End Sub

   Private Sub ShowAlertMessage(message As String)
      System.Windows.Browser.HtmlPage.Window.Alert(message)
   End Sub   

   Private Sub ShowBrowserMessage(message As String)
      outputBlock.Text += message + vbCrLf
   End Sub
End Module


The following example simplifies this code by instantiating the Action(Of T) delegate rather than explicitly defining a new delegate and assigning a named method to it.


Module Example
   Private outputBlock As System.Windows.Controls.TextBlock

   Public Sub Demo(outputBlock As System.Windows.Controls.TextBlock)
      Example.outputBlock = outputBlock
      Dim messageTarget As Action(Of String) 

      Dim rnd As New Random()
      If rnd.NextDouble() <= .5
         messageTarget = AddressOf ShowAlertMessage
      Else
         messageTarget = AddressOf ShowBrowserMessage
      End If
      messageTarget("Hello, World!")
   End Sub

   Private Sub ShowAlertMessage(message As String)
      System.Windows.Browser.HtmlPage.Window.Alert(message)
   End Sub   

   Private Sub ShowBrowserMessage(message As String)
      outputBlock.Text += message + vbCrLf
   End Sub
End Module


You can also use the Action(Of 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(Of T) delegate instance, as the following example illustrates.


Module Example
   Private outputBlock As System.Windows.Controls.TextBlock

   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Example.outputBlock = outputBlock
      Dim messageTarget As Action(Of String)

      Dim rnd As New Random()

      If (Rnd.NextDouble() <= 0.5) Then
         messageTarget = Sub(s) ShowAlertMessage(s)
      Else
         messageTarget = Sub(s) ShowBrowserMessage(s)
      End If

      messageTarget("Hello, World!")
   End Sub

   Private Sub ShowAlertMessage(ByVal message As String)
      System.Windows.Browser.HtmlPage.Window.Alert(message)
   End Sub

   Private Sub ShowBrowserMessage(ByVal message As String)
      outputBlock.Text += message + vbCrLf
   End Sub
End Module


The ForEach and ForEach(Of T) methods each take an Action(Of 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(Of T) delegate to print the contents of a List(Of 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(Of 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(Of T).ForEach method, whose single parameter is an Action(Of T) delegate. Similarly, in the C# example, an Action(Of T) delegate is not explicitly instantiated because the signature of the anonymous method matches the signature of the Action(Of T) delegate that is expected by the List(Of T).ForEach method.


Imports System.Collections.Generic

Class Example
   Private Shared outputBlock As System.Windows.Controls.TextBlock
   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

      Example.outputBlock = outputBlock
      Dim names As New List(Of 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(AddressOf Print)
   End Sub

   Shared Sub Print(ByVal s As String)
      outputBlock.Text &= s & vbCrLf
   End Sub
End Class

' This code will produce output similar to the following:
' 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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