You can use this delegate to pass a method as a parameter without explicitly declaring a custom delegate. The 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 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.) Typically, such a method is used to perform an operation.
.gif) Note: |
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To reference a method that has one parameter and returns a value, use the generic
Func<(Of <(T, TResult>)>) delegate instead.
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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.
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);
}
}
The following example simplifies this code by instantiating the Action<(Of <(T>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.
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);
}
}
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).)
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);
}
}
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Visual Basic requires that a lambda expression return a value. As a result, the
Action<(Of <(T>)>) delegate cannot be used with a lambda expression in Visual Basic.
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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.