Action Delegate

 
System_CAPS_noteNote

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Encapsulates a method that has no parameters and does not return a value.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

Public Delegate Sub Action

You can use this 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 no parameters and no return 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.

System_CAPS_noteNote

To reference a method that has no parameters and returns a value, use the generic 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 instead of 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();
   }
}

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 = Sub() name1.DisplayToWindow()
      methodCall()
   End Sub
End Module

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
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
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