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Action Delegate

Updated: February 2009

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

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

'Declaration
Public Delegate Sub Action
'Usage
Dim instance As New Action(AddressOf HandlerMethod)

You can use this delegate to pass a method that performs some operation as a parameter without explicitly declaring a custom delegate to encapsulate it. The encapsulated method must correspond to the method signature defined by this delegate. This means that the method must have no parameters and no return value.

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 

   Public Sub New(name As String)
      Me.instanceName = name
   End Sub 

   Public Sub DisplayToConsole()
      Console.WriteLine(Me.instanceName)
   End Sub    

   Public Sub DisplayToWindow()
      MsgBox(Me.instanceName)
   End Sub    
End Class 

Public Module testDelegate
   Public Sub Main()
      Dim testName As New Name("Koani")
      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 

   Public Sub New(name As String)
      Me.instanceName = name
   End Sub 

   Public Sub DisplayToConsole()
      Console.WriteLine(Me.instanceName)
   End Sub    

   Public Sub DisplayToWindow()
      MsgBox(Me.instanceName)
   End Sub    
End Class 

Public Module testDelegate
   Public Sub Main()
      Dim testName As New Name("Koani")
      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. (For an introduction to anonymous methods, see Anonymous Methods (C# Programming Guide).)

using System;
using System.Windows.Forms;

public class Name
{
   private string instanceName;

   public Name(string name)
   {
      this.instanceName = name;
   }

   public void DisplayToConsole()
   {
      Console.WriteLine(this.instanceName);
   }

   public void DisplayToWindow()
   {
      MessageBox.Show(this.instanceName);
   }
}

public class Anonymous
{
   public static void Main()
   {
      Name testName = new Name("Koani");
      Action showMethod = delegate() { testName.DisplayToWindow();} ;
      showMethod();
   }
}

In C#, you can also assign a lambda expression to an Action delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (C# Programming Guide).)

Public Class Name
   Private instanceName As String 

   Public Sub New(name As String)
      Me.instanceName = name
   End Sub 

   Public Function DisplayToConsole() As Integer
      Console.WriteLine(Me.instanceName)
      Return 0
   End Function 

   Public Function DisplayToWindow() As Integer 
      Return MsgBox(Me.instanceName)
   End Function       
End Class 

Module LambdaExpression
   Public Sub Main()
      Dim name1 As New Name("Koani")
      Dim methodCall As Action = Function() name1.DisplayToWindow()
      methodCall()
   End Sub 
End Module
NoteNote:

    Visual Basic requires that a lambda expression return a value. As a result, that return value must be discarded if the lambda expression is to be used with the Action delegate.

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

Date

History

Reason

February 2009

Modified the note about lambda expressions in Visual Basic, and added a Visual Basic example that uses a lambda expression.

Customer feedback.

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