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Action(Of T) Delegate

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

To browse the .NET Framework source code for this type, see the Reference Source.

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 Covariance and Contravariance in Generics.

Parameters

obj
Type: T

The parameter of the method that this delegate encapsulates.

NoteNote

To view the .NET Framework source code for this type, see the Reference Source. You can browse through the source code online, download the reference for offline viewing, and step through the sources (including patches and updates) during debugging; see instructions.

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 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 instead of 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 = Sub(s) ShowWindowsMessage(s) 
      Else
         messageTarget = Sub(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

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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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