Action Delegate

Definition

public delegate void Action();

public delegate void Action();

type Action = delegate of unit -> unit

Public Delegate Sub Action()

Remarks

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 F# the function or method must return unit. 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.

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.

using System;
using System.Windows.Forms;

public delegate void ShowValue();

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 testTestDelegate
{
 public static void Main()
 {
 Name testName = new Name("Koani");
 ShowValue showMethod = testName.DisplayToWindow;
 showMethod();
 }
}

open System.Windows.Forms

type ShowValue = delegate of unit -> unit

type Name(name) =
 member _.DisplayToConsole() = 
 printfn "%s" name

 member _.DisplayToWindow() = 
 MessageBox.Show name |> ignore

let testName = Name "Koani"

let showMethod = ShowValue testName.DisplayToWindow

showMethod.Invoke()

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.

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 testTestDelegate
{
 public static void Main()
 {
 Name testName = new Name("Koani");
 Action showMethod = testName.DisplayToWindow;
 showMethod();
 }
}

open System
open System.Windows.Forms

type Name(name) =
 member _.DisplayToConsole() =
 printfn "%s" name

 member _.DisplayToWindow() =
 MessageBox.Show name |> ignore

let testName = Name "Koani"

// unit -> unit functions and methods can be cast to Action.
let showMethod = Action testName.DisplayToWindow

showMethod.Invoke()

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.)

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#) or Lambda Expressions (F#).)

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 LambdaExpression
{
 public static void Main()
 {
 Name testName = new Name("Koani");
 Action showMethod = () => testName.DisplayToWindow();
 showMethod();
 }
}

open System
open System.Windows.Forms

type Name(name) =
 member _.DisplayToConsole() = 
 printfn "%s" name

 member _.DisplayToWindow() = 
 MessageBox.Show name |> ignore

let testName = Name "Koani"

let showMethod = Action(fun () -> testName.DisplayToWindow())

showMethod.Invoke()

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

See also

• Func<TResult>