Action(Of T) Delegate
Updated: May 2009
Encapsulates a method that takes a single parameter and does not return a value.
Assembly: mscorlib (in mscorlib.dll)
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 Sub…End Sub construct.) Typically, such a method is used to perform an operation.
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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
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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.
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