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Func(Of T1, T2, T3, T4, TResult) Delegate

Encapsulates a method that has four parameters and returns a value of the type specified by the TResult parameter.

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

'Declaration
Public Delegate Function Func(Of In T1, In T2, In T3, In T4, Out TResult) ( _
	arg1 As T1, _
	arg2 As T2, _
	arg3 As T3, _
	arg4 As T4 _
) As TResult

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.

In T4

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

Out TResult

The type of the return value of the method that this delegate encapsulates.

This type parameter is covariant. That is, you can use either the type you specified or any type that is more derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

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.

Return Value

Type: TResult
The return value of the method that this delegate encapsulates.

You can use this delegate to represent a method that can be passed 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, each of which is passed to it by value, and that it must return a value.

NoteNote

To reference a method that has four parameters and returns void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action(Of T1, T2, T3, T4) delegate instead.

When you use the Func(Of T1, T2, T3, T4, TResult) 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 generic delegate named Searcher and assigns a reference to the IndexOf method to its delegate instance.

Delegate Function Searcher(searchString As String, _
                           start As Integer,  _
                           count As Integer, _
                           type As StringComparison) As Integer 

Module DelegateExample
   Public Sub Main()
      Dim title As String = "The House of the Seven Gables" 
      Dim position As Integer = 0
      Dim finder As Searcher = AddressOf title.IndexOf
      Do 
         Dim characters As Integer = title.Length - position
         position = finder("the", position, characters, _
                         StringComparison.InvariantCultureIgnoreCase) 
         If position >= 0 Then
            position += 1
            Console.WriteLine("'The' found at position {0} in {1}.", _
                              position, title)
         End If    
      Loop While position > 0   
   End Sub 
End Module

The following example simplifies this code by instantiating the Func(Of T1, T2, T3, T4, TResult) delegate instead of explicitly defining a new delegate and assigning a named method to it.

Module DelegateExample
   Public Sub Main()
      Dim title As String = "The House of the Seven Gables" 
      Dim position As Integer = 0
      Dim finder As Func(Of String, Integer, Integer, StringComparison, Integer) _
                    = AddressOf title.IndexOf
      Do 
         Dim characters As Integer = title.Length - position
         position = finder("the", position, characters, _
                         StringComparison.InvariantCultureIgnoreCase) 
         If position >= 0 Then
            position += 1
            Console.WriteLine("'The' found at position {0} in {1}.", _
                              position, title)
         End If    
      Loop While position > 0   
   End Sub 
End Module

You can use the Func(Of T1, T2, T3, T4, TResult) 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 DelegateExample
{
   public static void Main()
   {
      string title = "The House of the Seven Gables";
      int position = 0;
      Func<string, int, int, StringComparison, int> finder = 
           delegate(string s, int pos, int chars, StringComparison type) 
           { return title.IndexOf(s, pos, chars, type); };
      do
      {
         int characters = title.Length - position;
         position = finder("the", position, characters, 
                         StringComparison.InvariantCultureIgnoreCase);
         if (position >= 0)
         {
            position++;
            Console.WriteLine("'The' found at position {0} in {1}.", 
                              position, title);
         }
      } while (position > 0);
   }
}

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

Module DelegateExample
   Public Sub Main()
      Dim title As String = "The House of the Seven Gables" 
      Dim position As Integer = 0
      Dim finder As Func(Of String, Integer, Integer, StringComparison, Integer) _
                    = Function(s, pos, chars, type) _
                    title.IndexOf(s, pos, chars, type)
      Do 
         Dim characters As Integer = title.Length - position
         position = finder("the", position, characters, _
                         StringComparison.InvariantCultureIgnoreCase) 
         If position >= 0 Then
            position += 1
            Console.WriteLine("'The' found at position {0} in {1}.", _
                              position, title)
         End If    
      Loop While position > 0   
   End Sub 
End Module

The underlying type of a lambda expression is one of the generic Func delegates. This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate. In particular, because many methods of types in the System.Linq namespace have Func parameters, you can pass these methods a lambda expression without explicitly instantiating a Func delegate.

The following example demonstrates how to declare and use a Func(Of T1, T2, TResult) delegate. This example declares a Func(Of T1, T2, TResult) variable and assigns it a lambda expression that takes a String value and an Int32 value as parameters. The lambda expression returns true if the length of the String parameter is equal to the value of the Int32 parameter. The delegate that encapsulates this method is subsequently used in a query to filter strings in an array of strings.

Imports System.Collections.Generic
Imports System.Linq

Public Module Func3Example

   Public Sub Main()
      Dim predicate As Func(Of String, Integer, Boolean) = Function(str, index) str.Length = index

      Dim words() As String = { "orange", "apple", "Article", "elephant", "star", "and" }
      Dim aWords As IEnumerable(Of String) = words.Where(predicate)

      For Each word As String In aWords
         Console.WriteLine(word)
      Next    
   End Sub 
End Module

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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