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Action(Of T) Delegate

Updated: May 2009

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 T) ( _
	obj As T _
)
'Usage
Dim instance As New Action(Of T)(AddressOf HandlerMethod)

Type Parameters

T

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

Parameters

obj
Type: T

The parameter of the method that this delegate encapsulates.

You can use this delegate to pass a method as a parameter without explicitly declaring a custom delegate. The 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 must not return a value. (In C#, the method must return void. In Visual Basic, it must be defined by the SubEnd Sub construct.) 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 TestCustomDelegate
   Public Sub Main
      Dim messageTarget As DisplayMessage 

      If Environment.GetCommandLineArgs().Length > 1 Then
         messageTarget = AddressOf ShowWindowsMessage
      Else
         messageTarget = AddressOf Console.WriteLine
      End If
      messageTarget("Hello, World!")
   End Sub 

   Private Sub ShowWindowsMessage(message As String)
      MsgBox(message)
   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 TestAction1
   Public Sub Main
      Dim messageTarget As Action(Of String) 

      If Environment.GetCommandLineArgs().Length > 1 Then
         messageTarget = AddressOf ShowWindowsMessage
      Else
         messageTarget = AddressOf Console.WriteLine
      End If
      messageTarget("Hello, World!")
   End Sub 

   Private Sub ShowWindowsMessage(message As String)
      MsgBox(message)
   End Sub    
End Module

You can also use the Action(Of T) 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 TestAnonMethod
{
   public static void Main()
   {
      Action<string> messageTarget; 

      if (Environment.GetCommandLineArgs().Length > 1)
         messageTarget = delegate(string s) { ShowWindowsMessage(s); };
      else
         messageTarget = delegate(string s) { Console.WriteLine(s); };

      messageTarget("Hello, World!");
   }

   private static void ShowWindowsMessage(string message)
   {
      MessageBox.Show(message);      
   }
}

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

Imports System.Windows.Forms

Public Module TestLambdaExpression
   Public Sub Main()
      Dim messageTarget As Action(Of String) 

      If Environment.GetCommandLineArgs().Length > 1 Then
         messageTarget = Function(s) ShowWindowsMessage(s) 
      Else
         messageTarget = Function(s) ShowConsoleMessage(s)
      End If
      messageTarget("Hello, World!")
   End Sub 

   Private Function ShowWindowsMessage(message As String) As Integer 
      Return MessageBox.Show(message)      
   End Function 

   Private Function ShowConsoleMessage(message As String) As Integer
      Console.WriteLine(message)
      Return 0
   End Function 
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(Of T) delegate.

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
Imports System.Collections.Generic

Class Program
    Shared Sub Main()
        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)
        Console.WriteLine(s)
    End Sub 
End Class 

' This code will produce output similar to the following: 
' Bruce 
' Alfred 
' Tim 
' Richard

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, 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, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

May 2009

Expanded the remarks about the example.

Customer feedback.

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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