Action Delegate
Updated: February 2009
Encapsulates a method that takes no parameters and does not return a value.
Assembly: System.Core (in System.Core.dll)
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.
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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
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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 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.
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