Action Delegate (System)

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.NET Framework Class Library

Action Delegate

[This documentation is for preview only, and is subject to change in later releases. Blank topics are included as placeholders.]

Encapsulates a method that has no parameters and does not return a value.

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

Syntax

Visual Basic


Public Delegate Sub Action


public delegate void Action()

Visual C++


public delegate void Action()


type Action = 
    delegate of unit -> unit

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 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 no parameters and returns a value, use the generic Func<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.

Visual Basic


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


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();
   }
}

Visual C++


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

public delegate void ShowValue();


public ref class Name
{
private:
   String^ instanceName;

public:
   Name(String^ name) 
   {
      instanceName = name;
   }

   void DisplayToConsole()
   {
      Console::WriteLine(this->instanceName);
   }

   void DisplayToWindow()
   {
      MessageBox::Show(this->instanceName);
   }
};

int main()
{
   Name^ testName = gcnew Name(L"Koani");
   ShowValue^ showMethod;
   showMethod = gcnew ShowValue(testName, &Name::DisplayToWindow);
   showMethod();
   return 0;
}

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.

Visual Basic


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


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();
   }
}

Visual C++


#using <System.Windows.Forms.dll>

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

public ref class Name
{
private:
   String^ instanceName;

public:
   Name(String^ name)
   {
      instanceName = name;
   }

   void DisplayToConsole()
   {
      Console::WriteLine(this->instanceName);
   }

   void DisplayToWindow()
   {
      MessageBox::Show(this->instanceName);
   }
};


int main()
{
   Name^ testName = gcnew Name(L"Koani");
   System::Action^ showMethod;
   showMethod += gcnew Action(testName, &Name::DisplayToWindow);
   showMethod();
   return 0;
}

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();
   }
}

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).)

Visual Basic


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


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();
   }
}

Version Information

.NET Framework

Supported in: 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Platforms

Windows 8 Release Preview, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 SP2, Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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<TResult>