Action<T> Delegate

Definition

generic <typename T>
public delegate void Action(T obj);

public delegate void Action<in T>(T obj);

public delegate void Action<in T>(T obj) where T : allows ref struct;

public delegate void Action<T>(T obj);

type Action<'T> = delegate of 'T -> unit

Public Delegate Sub Action(Of In T)(obj As T)

Public Delegate Sub Action(Of T)(obj As T)

Type Parameters

Parameters

Examples

The following example demonstrates the use of the Action<T> delegate to print the contents of a List<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<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<T>.ForEach method, whose single parameter is an Action<T> delegate. Similarly, in the C# example, an Action<T> delegate is not explicitly instantiated because the signature of the anonymous method matches the signature of the Action<T> delegate that is expected by the List<T>.ForEach method.

List<string> names = new List<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(Print);

// The following demonstrates the anonymous method feature of C#
// to display the contents of the list to the console.
names.ForEach(delegate(string name)
{
 Console.WriteLine(name);
});

void Print(string s)
{
 Console.WriteLine(s);
}

/* This code will produce output similar to the following:
* Bruce
* Alfred
* Tim
* Richard
* Bruce
* Alfred
* Tim
* Richard
*/

// F# provides a type alias for System.Collections.List<'T> as ResizeArray<'T>.
let names = ResizeArray<string>()
names.Add "Bruce"
names.Add "Alfred"
names.Add "Tim"
names.Add "Richard"

let print s = printfn "%s" s

// Display the contents of the list using the print function.
names.ForEach(Action<string> print)

// The following demonstrates the lambda expression feature of F#
// to display the contents of the list to the console.
names.ForEach(fun s -> printfn "%s" s)

(* This code will produce output similar to the following:
* Bruce
* Alfred
* Tim
* Richard
* Bruce
* Alfred
* Tim
* Richard
*)

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

Remarks

You can use the Action<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.

When you use the Action<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.

using System;
using System.Windows.Forms;

delegate void DisplayMessage(string message);

public class TestCustomDelegate
{
 public static void Main()
 {
 DisplayMessage messageTarget;

 if (Environment.GetCommandLineArgs().Length > 1)
 messageTarget = ShowWindowsMessage;
 else
 messageTarget = Console.WriteLine;

 messageTarget("Hello, World!");
 }

 private static void ShowWindowsMessage(string message)
 {
 MessageBox.Show(message);
 }
}

open System
open System.Windows.Forms

type DisplayMessage = delegate of message: string -> unit

let showWindowsMessage message = 
 MessageBox.Show message |> ignore

let messageTarget =
 DisplayMessage(
 if Environment.GetCommandLineArgs().Length > 1 then
 showWindowsMessage
 else
 printfn "%s"
 )

messageTarget.Invoke "Hello, World!"

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<T> delegate instead of explicitly defining a new delegate and assigning a named method to it.

using System;
using System.Windows.Forms;

public class TestAction1
{
 public static void Main()
 {
 Action<string> messageTarget;

 if (Environment.GetCommandLineArgs().Length > 1)
 messageTarget = ShowWindowsMessage;
 else
 messageTarget = Console.WriteLine;

 messageTarget("Hello, World!");
 }

 private static void ShowWindowsMessage(string message)
 {
 MessageBox.Show(message);
 }
}

open System
open System.Windows.Forms

let showWindowsMessage message = 
 MessageBox.Show message |> ignore

let messageTarget =
 Action<string>(
 if Environment.GetCommandLineArgs().Length > 1 then
 showWindowsMessage
 else
 printfn "%s"
 )

messageTarget.Invoke "Hello, World!"

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<T> 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 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<T> delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions.)

using System;
using System.Windows.Forms;

public class TestLambdaExpression
{
 public static void Main()
 {
 Action<string> messageTarget;

 if (Environment.GetCommandLineArgs().Length > 1)
 messageTarget = s => ShowWindowsMessage(s);
 else
 messageTarget = s => Console.WriteLine(s);

 messageTarget("Hello, World!");
 }

 private static void ShowWindowsMessage(string message)
 {
 MessageBox.Show(message);
 }
}

open System
open System.Windows.Forms

let showWindowsMessage message = 
 MessageBox.Show message |> ignore

let messageTarget =
 Action<string>(
 if Environment.GetCommandLineArgs().Length > 1 then
 fun s -> showWindowsMessage s
 else
 fun s -> printfn "%s" s
 )

messageTarget.Invoke "Hello, World!"

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 methods each take an Action<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.

See also

• Func<T,TResult>