Action Delegate (System)

'Usage

Dim instance As New Action(AddressOf HandlerMethod)

'Declaration

Public Delegate Sub Action

Remarks

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.

Note:
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).)