Action<T> Delegate

Definition

Namespace:

System

Assembly:

System.Runtime.dll

Encapsulates a method that has a single parameter and does not return a value.

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

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

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

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

Type Parameters

T

The type of the parameter of the method that this delegate encapsulates.

This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

Parameters

obj

T

The parameter of the method that this delegate encapsulates.

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.

Note

To reference a method that has one parameter and returns a value, use the generic Func<T,TResult> delegate instead.

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.Windows.Forms.dll>

using namespace System;
using namespace System::Windows::Forms;

public delegate void DisplayMessage(String^ message);

public ref class TestCustomDelegate
{
public:
   static void ShowWindowsMessage(String^ message)
   {
      MessageBox::Show(message);      
   }
};

int main()
{
    DisplayMessage^ messageTarget; 
      
    if (Environment::GetCommandLineArgs()->Length > 1)
       messageTarget = gcnew DisplayMessage(&TestCustomDelegate::ShowWindowsMessage);
    else
       messageTarget = gcnew DisplayMessage(&Console::WriteLine);
    
    messageTarget(L"Hello World!");
    return 0;
}

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.Windows.Forms.dll>

using namespace System;
using namespace System::Windows::Forms;

namespace ActionExample
{
   public ref class Message
   {
   public:
      static void ShowWindowsMessage(String^ message)
      {
         MessageBox::Show(message);
      }
   };
}

int main()
{
   Action<String^>^ messageTarget;

   if (Environment::GetCommandLineArgs()->Length > 1)
      messageTarget = gcnew Action<String^>(&ActionExample::Message::ShowWindowsMessage);
   else
      messageTarget = gcnew Action<String^>(&Console::WriteLine);

   messageTarget("Hello, World!");
   return 0;
}

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.

Extension Methods

GetMethodInfo(Delegate)

Gets an object that represents the method represented by the specified delegate.

See also

• Func<T,TResult>