Action Delegate

Updated: October 2010

Encapsulates a method that takes no parameters and does not return a value.

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

'Declaration
<TypeForwardedFromAttribute("System.Core, Version=2.0.5.0, Culture=Neutral, PublicKeyToken=7cec85d7bea7798e")> _
Public Delegate Sub Action

You can use the Action 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 no parameters and no return 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 no parameters and that returns a value, use the Func(Of TResult) delegate instead.

When you use the Action delegate, you do not have to explicitly define a delegate that encapsulates a parameterless procedure. For example, the following code explicitly declares a delegate named ShowValue and assigns a reference to the Name.DisplayToWindow instance method to its delegate instance.


Public Delegate Sub ShowValue()

Public Class Name
   Private instanceName As String
   Private outputBlock As System.Windows.Controls.TextBlock

   Public Sub New(ByVal name As String, _
                  ByVal outBlock As System.Windows.Controls.TextBlock)
      Me.instanceName = name
      Me.outputBlock = outBlock
   End Sub

   Public Sub DisplayToConsole()
      outputBlock.Text &= Me.instanceName & vbCrLf
   End Sub

   Public Sub DisplayToWindow()
      System.Windows.Browser.HtmlPage.Window.Alert(Me.instanceName)
   End Sub
End Class

Public Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim testName As New Name("Koani", outputBlock)
      Dim showMethod As ShowValue = AddressOf testName.DisplayToWindow
      showMethod()
   End Sub
End Module


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


Public Class Name
   Private instanceName As String
   Private outputBlock As System.Windows.Controls.TextBlock

   Public Sub New(ByVal name As String, _
                  ByVal outBlock As System.Windows.Controls.TextBlock)
      Me.instanceName = name
      Me.outputBlock = outBlock
   End Sub

   Public Sub DisplayToConsole()
      outputBlock.Text &= Me.instanceName & vbCrLf
   End Sub

   Public Sub DisplayToWindow()
      System.Windows.Browser.HtmlPage.Window.Alert(Me.instanceName)
   End Sub
End Class

Public Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim testName As New Name("Koani", outputBlock)
      Dim showMethod As Action = AddressOf testName.DisplayToWindow
      showMethod()
   End Sub
End Module


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


using System;

public class Name
{
   private string instanceName;
   private System.Windows.Controls.TextBlock outputBlock;

   public Name(string name, System.Windows.Controls.TextBlock outBlock)
   {
      this.instanceName = name;
      this.outputBlock = outBlock;
   }

   public void DisplayToConsole()
   {
      outputBlock.Text += this.instanceName + "\n";
   }

   public void DisplayToWindow()
   {
      System.Windows.Browser.HtmlPage.Window.Alert(this.instanceName);
   }
}

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Name testName = new Name("Koani", outputBlock);
      Action showMethod = delegate() { testName.DisplayToWindow(); };
      showMethod();
   }
}


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


Public Class Name
   Private instanceName As String
   Private outputBlock As System.Windows.Controls.TextBlock

   Public Sub New(name As String, outBlock As System.Windows.Controls.TextBlock)
      Me.instanceName = name
      Me.outputBlock = outBlock
   End Sub

   Public Sub DisplayToBrowser()
      outputBlock.Text += Me.instanceName + vbCrLf
   End Sub

   Public Sub DisplayToWindow()
      System.Windows.Browser.HtmlPage.Window.Alert(Me.instanceName)
   End Sub
End Class

Module Example
  Public Sub Demo(outputBlock As System.Windows.Controls.TextBlock)
      Dim testName As New Name("Koani", outputBlock)
      Dim showMethod As Action = Sub() testName.DisplayToWindow()
      showMethod()
   End Sub
End Module


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.

Community Additions

ADD
Show: