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.
Assembly: mscorlib (in mscorlib.dll)
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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 Sub…End 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.
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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 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
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
