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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:  mscorlib (in mscorlib.dll)

'Declaration
<TypeForwardedFromAttribute("System.Core, Version=2.0.5.0, Culture=Neutral, PublicKeyToken=7cec85d7bea7798e")> _
Public Delegate Function Func(Of In T, Out TResult) ( _
	arg As T _
) As TResult

Type Parameters

In T

The type of the 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 2678dc63-c7f9-4590-9ddc-0a4df684d42e.

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 2678dc63-c7f9-4590-9ddc-0a4df684d42e.

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 Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' Instantiate delegate to reference UppercaseString method
      Dim convertMeth As ConvertMethod = AddressOf UppercaseString
      Dim name As String = "Dakota"
      ' Use delegate instance to call UppercaseString method
      outputBlock.Text &= convertMeth(name) & vbCrLf
   End Sub

   Private Function UppercaseString(ByVal 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 Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' 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
      outputBlock.Text &= convertMethod(name) & vbCrLf
   End Sub

   Private Function UppercaseString(ByVal 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.


using System;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Func<string, string> convert = delegate(string s)
         { return s.ToUpper(); };

      string name = "Dakota";
      outputBlock.Text += convert(name) + "\n";
   }
}


You can also assign a lambda expression to a Func(Of T, TResult) delegate, as the following example illustrates.


Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim convert As Func(Of String, String) = Function(s) s.ToUpper()

      Dim name As String = "Dakota"
      outputBlock.Text &= convert(name) & vbCrLf
   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 Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' Declare a Example 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.
      For Each word As String In aWords
         outputBlock.Text &= word & vbCrLf
      Next
   End Sub
End Module
' This code example produces the following output:
'           
'   ORANGE
'   APPLE
'   ARTICLE
'   ELEPHANT


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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