Action(T) Delegate (System)

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Microsoft Visual Studio 2010/.NET Framework 4

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.NET Framework Class Library

Action<(Of <(T>)>) Delegate

Updated: June 2010

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

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

Syntax

Visual Basic

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

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

Visual C++

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

type Action = 
    delegate of 
        obj:'T -> unit

Type Parameters

in In in in 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: Type: T
The parameter of the method that this delegate encapsulates.

Remarks

You can use the Action<(Of <(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<(Of <(T, TResult>)>) delegate instead.

When you use the Action<(Of <(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.

Visual Basic

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

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);      
   }
}

Visual C++

#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;
}

The following example simplifies this code by instantiating the Action<(Of <(T>)>) delegate instead of explicitly defining a new delegate and assigning a named method to it.

Visual Basic

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

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);      
   }
}

Visual C++

#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;
}

You can also use the Action<(Of <(T>)>) 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 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<(Of <(T>)>) delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (C# Programming Guide).)

Visual Basic

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

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);      
   }
}

The ForEach and ForEach<(Of <(T>)>) methods each take an Action<(Of <(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.

Examples

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

Visual Basic

Imports System
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

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        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);
        });
    }

    private static void Print(string s)
    {
        Console.WriteLine(s);
    }
}
/* This code will produce output similar to the following:
 * Bruce
 * Alfred
 * Tim
 * Richard
 * Bruce
 * Alfred
 * Tim
 * Richard
 */

Version Information

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Platforms

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference

System Namespace

Func<(Of <(T, TResult>)>)

Change History

Date	History	Reason
June 2010	Modified Visual Basic lambda expression to use Sub keyword.	Customer feedback.

Date

History

Reason

June 2010

Modified Visual Basic lambda expression to use Sub keyword.

Customer feedback.

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Anotaciones

Additional example

Morten Wennevik | Editar | Mostrar historial

The following code combines a reporter interface and an Action<IReport> to demonstrate how paramters are passed around

using System;
using System.Threading;
 
namespace MyApplication
{
    public class Program
    {
        static void Main()
        {
            var measurer = new Measurer();
            measurer.Measure((reporter) =>
                {
                    reporter.WriteLine("Starting work ...");
                    Work(1000, reporter);
                }, true);
        }
 
        static void Work(int duration, IReport reporter)
        {
            reporter.WriteLine("Going to sleep ...");
            Thread.Sleep(duration);
            reporter.WriteLine("Waking up ...");
            throw new TimeoutException();
        }
    }
 
    public interface IReport
    {
        void WriteLine(string text, params object[] arguments);
    }
 
    public class ConsoleReporter : IReport
    {
        public void WriteLine(string text, params object[] arguments)
        {
            Console.WriteLine(text, arguments);
        }
    }
 
    public class Measurer
    {
        public void Measure(Action<IReport> action, bool trapException)
        {
            var start = DateTime.Now;
            var reporter = new ConsoleReporter();
            try { action(reporter); }
            catch (Exception ex)
            {
                if (trapException)
                    reporter.WriteLine("Action failed with a {0}: {1}", ex.GetType().Name, ex.Message);
                else
                    throw;
            }
            reporter.WriteLine("Action executed in {0:f2} seconds", (DateTime.Now - start).TotalSeconds);
        }
    }
}
 
/* Output: * Starting work ... * Going to sleep ... * Waking up ... * Action failed with a TimeoutException: The operation has timed out. * Action executed in 1,01 seconds */

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