Func(Of T1, T2, TResult) Delegate

 

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

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

Public Delegate Function Func(Of In T1, In T2, Out TResult) (
	arg1 As T1,
	arg2 As T2
) As TResult

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.

Return Value

Type: TResult

The return value of the method that this delegate encapsulates.

Type Parameters

InT1

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

InT2

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

OutTResult

The type of 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 two parameters, each of which is passed to it by value, and that it must return a value.

System_CAPS_noteNote

To reference a method that has two 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) delegate instead.

When you use the Func(Of T1, T2, TResult) delegate you do not have to explicitly define a delegate that encapsulates a method with two parameters. For example, the following code explicitly declares a delegate named ExtractMethod and assigns a reference to the ExtractWords method to its delegate instance.

' Declare a delegate to represent string extraction method
Delegate Function ExtractMethod(ByVal stringToManipulate As String, _
                                ByVal maximum As Integer) As String()

Module DelegateExample
   Public Sub Main()
      ' Instantiate delegate to reference ExtractWords method
      Dim extractMeth As ExtractMethod = AddressOf ExtractWords
      Dim title As String = "The Scarlet Letter"
      ' Use delegate instance to call ExtractWords method and display result
      For Each word As String In extractMeth(title, 5)
         Console.WriteLine(word)
      Next   
   End Sub

   Private Function ExtractWords(phrase As String, limit As Integer) As String()
      Dim delimiters() As Char = {" "c}
      If limit > 0 Then
         Return phrase.Split(delimiters, limit)
      Else
         Return phrase.Split(delimiters)
      End If
   End Function
End Module

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

Module GenericFunc
   Public Sub Main()
      ' Instantiate delegate to reference ExtractWords method
      Dim extractMeth As Func(Of String, Integer, String()) = AddressOf ExtractWords
      Dim title As String = "The Scarlet Letter"
      ' Use delegate instance to call ExtractWords method and display result
      For Each word As String In extractMeth(title, 5)
         Console.WriteLine(word)
      Next   
   End Sub

   Private Function ExtractWords(phrase As String, limit As Integer) As String()
      Dim delimiters() As Char = {" "c}
      If limit > 0 Then
         Return phrase.Split(delimiters, limit)
      Else
         Return phrase.Split(delimiters)
      End If
   End Function
End Module

You can use the Func(Of T1, T2, 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 Anonymous
{
   public static void Main()
   {
      Func<string, int, string[]> extractMeth = delegate(string s, int i)
         { char[] delimiters = new char[] {' '}; 
           return i > 0 ? s.Split(delimiters, i) : s.Split(delimiters);
         };

      string title = "The Scarlet Letter";
      // Use Func instance to call ExtractWords method and display result
      foreach (string word in extractMeth(title, 5))
         Console.WriteLine(word);
   }
}

You can also assign a lambda expression to a Func(Of T1, T2, 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 LambdaExpression
   Public Sub Main()
      Dim separators() As Char = {" "c}
      Dim extract As Func(Of String, Integer, String()) = Function(s, i) _
          CType(iif(i > 0, s.Split(separators, i), s.Split(separators)), String())  

      Dim title As String = "The Scarlet Letter"
      For Each word As String In extract(title, 5)
         Console.WriteLine(word)
      Next   
   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(Of T1, T2, TResult) parameters, you can pass these methods a lambda expression without explicitly instantiating a Func(Of T1, T2, TResult) 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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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