Action<T> Delegate
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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);
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)
- 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.- obj
- T
The parameter of the method that this delegate encapsulates.
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
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.
Get |
Gets an object that represents the method represented by the specified delegate. |
Product | Versions |
---|---|
.NET | Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9 |
.NET Framework | 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1 |
.NET Standard | 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 2.0, 2.1 |
UWP | 10.0 |
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