You can use this delegate to pass a method that performs some operation as a parameter without explicitly declaring a custom delegate to encapsulate it. The encapsulated method must correspond to the method signature defined by this delegate. This means that the method must have no parameters and no return value.
.gif) Note: |
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To reference a method that has no parameters and that returns a value, use the Func<(Of <(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.
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();
}
}
The following example simplifies this code by instantiating the Action delegate rather than explicitly defining a new delegate and assigning a named method to it.
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();
}
}
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();
}
}
In C#, 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).)
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 = Function() 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();
}
}
| Note: |
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Visual Basic requires that a lambda expression return a value. As a result, that return value must be discarded if the lambda expression is to be used with the Action delegate. |