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Action(Of T1, T2, T3) Delegate

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

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

'Declaration
Public Delegate Sub Action(Of In T1, In T2, In T3) ( _
	arg1 As T1, _
	arg2 As T2, _
	arg3 As T3 _
)

Type Parameters

In T1

The type of the first 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.

In T2

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

In T3

The type of the third 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.

You can use the Action(Of T1, T2, T3) 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 three 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. It can also be a method that returns a value that is ignored.) Typically, such a method is used to perform an operation.

NoteNote

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

When you use the Action(Of T1, T2, T3) delegate, you do not have to explicitly define a delegate that encapsulates a method with three 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)

Module TestDelegate
   Public Sub Main()
      Dim ordinals() As String = {"First", "Second", "Third", "Fourth", "Fifth"}
      Dim copiedOrdinals(ordinals.Length - 1) As String 
      Dim copyOperation As StringCopy = AddressOf CopyStrings
      copyOperation(ordinals, copiedOrdinals, 3)
      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)
      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 source.Length - 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 instead of explicitly defining a new delegate and assigning a named method to it.

Module TestAction3
   Public Sub Main()
      Dim ordinals() As String = {"First", "Second", "Third", "Fourth", "Fifth"}
      Dim copiedOrdinals(ordinals.Length - 1) As String 
      Dim copyOperation As Action(Of String(), String(), Integer) = AddressOf CopyStrings
      copyOperation(ordinals, copiedOrdinals, 3)
      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)
      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 source.Length - 1
         target(ctr) = String.Copy(source(ctr))
      Next 
   End Sub 
End Module

You can also use the Action(Of T1, T2, T3) 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 TestAnon
{
   public static void Main()
   {
      string[] ordinals = {"First", "Second", "Third", "Fourth", "Fifth"};
      string[] copiedOrdinals = new string[ordinals.Length];           
      Action<string[], string[], int> copyOperation = delegate(string[] s1, 
                                                               string[] s2, 
                                                               int pos) 
                                      { CopyStrings(s1, s2, pos); };
      copyOperation(ordinals, copiedOrdinals, 3);
      foreach (string ordinal in copiedOrdinals)
         Console.WriteLine(String.IsNullOrEmpty(ordinal) ? "<None>" : ordinal);
   }

   private static void CopyStrings(string[] source, string[] target, int startPos)
   {
      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 <= source.Length - 1; ctr++)
         target[ctr] = String.Copy(source[ctr]);
   }
}

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

Public Module TestLambda
   Public Sub Main()
      Dim ordinals() As String = {"First", "Second", "Third", "Fourth", "Fifth"}
      Dim copiedOrdinals(ordinals.Length - 1) As String            
      Dim copyOperation As Action(Of String(), String(), Integer) = _
                           Sub(s1, s2, pos) CopyStrings(s1, s2, pos) 
      copyOperation(ordinals, copiedOrdinals, 3)
      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) 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 source.Length - 1 
         target(ctr) = String.Copy(source(ctr))
      Next 
      Return source.Length - startPos 
   End Function 
End Module 
' The example displays the following output: 
'       <None> 
'       <None> 
'       <None> 
'       Fourth 
'       Fifth

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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