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Action(Of T1, T2, T3, T4) Delegate

Updated: February 2009

Encapsulates a method that has four parameters and does not return a value.

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

'Declaration
Public Delegate Sub Action(Of T1, T2, T3, T4) ( _
	arg1 As T1, _
	arg2 As T2, _
	arg3 As T3, _
	arg4 As T4 _
)
'Usage
Dim instance As New Action(Of T1, T2, T3, T4)(AddressOf HandlerMethod)

Type Parameters

T1

The type of the first parameter of the method that this delegate encapsulates.

T2

The type of the second parameter of the method that this delegate encapsulates.

T3

The type of the third parameter of the method that this delegate encapsulates.

T4

The type of the fourth parameter of the method that this delegate encapsulates.

Parameters

arg1
Type: T1

The first parameter of the method that this delegate encapsulates.

arg2
Type: T2

The second parameter of the method that this delegate encapsulates.

arg3
Type: T3

The third parameter of the method that this delegate encapsulates.

arg4
Type: T4

The fourth parameter of the method that this delegate encapsulates.

You can use the Action(Of T1, T2, T3, T4) 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 four parameters that are all 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 SubEnd Sub construct.) Typically, such a method is used to perform an operation.

NoteNote:

To reference a method that has four parameters and returns a value, use the generic Func(Of T1, T2, T3, T4, TResult) delegate instead.

When you use the Action(Of T1, T2, T3, T4) delegate, you do not have to explicitly define a delegate that encapsulates a method with four parameters. For example, the following code explicitly declares a delegate named StringCopy and assigns a reference to the CopyStrings method to its delegate instance.

Delegate Sub StringCopy(stringArray1() As String, _
                        stringArray2() As String, _
                        indexToStart As Integer, _
                        numberToCopy As Integer)

Module TestDelegate
   Public Sub Main()
      Dim ordinals() As String = {"First", "Second", "Third", "Fourth", _
                                  "Fifth", "Sixth", "Seventh", "Eighth", _
                                  "Ninth", "Tenth"}
      Dim copiedOrdinals(ordinals.Length - 1) As String 
      Dim copyOperation As StringCopy = AddressOf CopyStrings
      copyOperation(ordinals, copiedOrdinals, 3, 5)
      For Each ordinal As String In copiedOrdinals
         Console.WriteLine(ordinal)
      Next     
   End Sub 

   Private Sub CopyStrings(source() As String, target() As String, _
                           startPos As Integer, number As Integer)
      If source.Length <> target.Length Then  
         Throw New IndexOutOfRangeException("The source and target arrays" & _
                   " must have the same number of elements.")
      End If 
      For ctr As Integer = startPos to startpos + number  - 1
         target(ctr) = String.Copy(source(ctr))
      Next 
   End Sub 
End Module

The following example simplifies this code by instantiating the Action(Of T1, T2, T3) delegate rather than explicitly defining a new delegate and assigning a named method to it.

Module TestAction4
   Public Sub Main()
      Dim ordinals() As String = {"First", "Second", "Third", "Fourth", _
                                  "Fifth", "Sixth", "Seventh", "Eighth", _
                                  "Ninth", "Tenth"}
      Dim copiedOrdinals(ordinals.Length - 1) As String 
      Dim copyOperation As Action(Of String(), String(), Integer, Integer) = _
                           AddressOf CopyStrings
      copyOperation(ordinals, copiedOrdinals, 3, 5)
      For Each ordinal As String In copiedOrdinals
         Console.WriteLine(ordinal)
      Next     
   End Sub 

   Private Sub CopyStrings(source() As String, target() As String, _
                           startPos As Integer, number As Integer)
      If source.Length <> target.Length Then  
         Throw New IndexOutOfRangeException("The source and target arrays" & _
                   " must have the same number of elements.")
      End If 
      For ctr As Integer = startPos to startpos + number  - 1
         target(ctr) = String.Copy(source(ctr))
      Next 
   End Sub 
End Module

You can also use the Action(Of T1, T2, T3, T4) 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;

public class TestAnonymousMethod
{
   public static void Main()
   {
      string[] ordinals = {"First", "Second", "Third", "Fourth", "Fifth", 
                           "Sixth", "Seventh", "Eighth", "Ninth", "Tenth"};
      string[] copiedOrdinals = new string[ordinals.Length];           
      Action<string[], string[], int, int> copyOperation = 
                                           delegate(string[] s1, string[] s2, 
                                           int pos, int num) 
                                { CopyStrings(s1, s2, pos, num); };  
      copyOperation(ordinals, copiedOrdinals, 3, 5);
      foreach (string ordinal in copiedOrdinals)
         Console.WriteLine(String.IsNullOrEmpty(ordinal) ? "<None>" : ordinal);
   }

   private static void CopyStrings(string[] source, string[] target, 
                                   int startPos, int number)
   {
      if (source.Length != target.Length) 
         throw new IndexOutOfRangeException("The source and target arrays must have the same number of elements.");

      for (int ctr = startPos; ctr <= startPos + number - 1; ctr++)
         target[ctr] = String.Copy(source[ctr]);
   }
}

You can also assign a lambda expression to an Action(Of T1, T2, T3, T4) delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (C# Programming Guide).)

Public Module TestLambdaExpression
   Public Sub Main()
      Dim ordinals() As String = {"First", "Second", "Third", "Fourth", "Fifth", _ 
                           "Sixth", "Seventh", "Eighth", "Ninth", "Tenth"}
      Dim copiedOrdinals(ordinals.Length - 1) As String            
      Dim copyOperation As Action(Of String(), String(), Integer, Integer) = _
                           Function(s1, s2, pos, num) CopyStrings(s1, s2, pos, num)  
      copyOperation(ordinals, copiedOrdinals, 3, 5)
      For Each ordinal As String In copiedOrdinals
         If String.IsNullOrEmpty(ordinal) Then
            Console.WriteLine("<None>")
         Else
            Console.WriteLine(ordinal)
         End If 
      Next    
   End Sub 

   Private Function CopyStrings(source() As String, target() As String, _ 
                                startPos As Integer, number As Integer) As Integer 
      If source.Length <> target.Length Then 
         throw new IndexOutOfRangeException("The source and target arrays must have the same number of elements.")
      End If 

      For ctr As Integer = startPos To startPos + number - 1
         target(ctr) = String.Copy(source(ctr))
      Next 
      Return number
   End Function 
End Module 
' The example displays the following output: 
'       <None> 
'       <None> 
'       <None> 
'       Fourth 
'       Fifth 
'       Sixth 
'       Seventh 
'       Eighth 
'       <None> 
'       <None>
NoteNote:

    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(Of T1, T2, T3, T4) delegate.

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

Date

History

Reason

February 2009

Modified the note about lambda expressions in Visual Basic, and added a Visual Basic example that uses a lambda expression.

Customer feedback.

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