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Func(Of T, TResult) Delegate

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

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

'Declaration
Public Delegate Function Func(Of T, TResult) ( _
	arg As T _
) As TResult
'Usage
Dim instance As New Func(Of T, TResult)(AddressOf HandlerMethod)

Type Parameters

T

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

TResult

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

Parameters

arg
Type: T

The 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 method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have one parameter that is passed to it by value and must return a value.

NoteNote:

To reference a method that has one parameter and returns void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action(Of T) delegate instead.

When you use the Func(Of T, TResult) delegate, you do not have to explicitly define a delegate that encapsulates a method with a single parameter. For example, the following code explicitly declares a delegate named ConvertMethod and assigns a reference to the UppercaseString method to its delegate instance.

' Declare a delegate to represent string conversion method 
Delegate Function ConvertMethod(ByVal inString As String) As String 

Module DelegateExample
   Public Sub Main()
      ' Instantiate delegate to reference UppercaseString method 
      Dim convertMeth As ConvertMethod = AddressOf UppercaseString
      Dim name As String = "Dakota" 
      ' Use delegate instance to call UppercaseString method
      Console.WriteLine(convertMeth(name))
   End Sub 

   Private Function UppercaseString(inputString As String) As String 
      Return inputString.ToUpper()
   End Function 
End Module

The following example simplifies this code by instantiating the Func(Of T, TResult) delegate rather than explicitly defining a new delegate and assigning a named method to it.

Module GenericFunc
   Public Sub Main()
      ' Instantiate delegate to reference UppercaseString method 
      Dim convertMethod As Func(Of String, String) = AddressOf UppercaseString
      Dim name As String = "Dakota" 
      ' Use delegate instance to call UppercaseString method
      Console.WriteLine(convertMethod(name))
   End Sub 

   Private Function UppercaseString(inputString As String) As String 
      Return inputString.ToUpper()
   End Function 
End Module

You can also use the Func(Of T, 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, string> convert = delegate(string s)
         { return s.ToUpper();}; 

      string name = "Dakota";
      Console.WriteLine(convert(name));   
   }
}

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

Module LambdaExpression
   Public Sub Main()
      Dim convert As Func(Of String, String) = Function(s) s.ToUpper()

      Dim name As String = "Dakota"
      Console.WriteLine(convert(name))  
   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 T, TResult) parameters, you can pass these methods a lambda expression without explicitly instantiating a Func(Of T, TResult) delegate.

The following example demonstrates how to declare and use a Func(Of T, TResult) delegate. This example declares a Func(Of T, TResult) variable and assigns it a lambda expression that converts the characters in a string to uppercase. The delegate that encapsulates this method is subsequently passed to the Select method to change the strings in an array of strings to uppercase.

Imports System.Collections.Generic
Imports System.Linq

Module Func
   Public Sub Main()
      ' Declare a Func variable and assign a lambda expression to the   
      ' variable. The method takes a string and converts it to uppercase. 
      Dim selector As Func(Of String, String) = Function(str) str.ToUpper()

      ' Create an array of strings. 
      Dim words() As String = { "orange", "apple", "Article", "elephant" }
      ' Query the array and select strings according to the selector method. 
      Dim aWords As IEnumerable(Of String) = words.Select(selector)

      ' Output the results to the console. 
      For Each word As String In aWords
         Console.WriteLine(word)
      Next 
   End Sub 
End Module 
' This code example produces the following output: 
'            
'   ORANGE 
'   APPLE 
'   ARTICLE 
'   ELEPHANT

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

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