Func Delegate

Definition

generic 
public delegate TResult Func(T1 arg1, T2 arg2);

public delegate TResult Func(T1 arg1, T2 arg2);

public delegate TResult Func(T1 arg1, T2 arg2) where T1 : allows ref struct where T2 : allows ref struct where TResult : allows ref struct;

public delegate TResult Func(T1 arg1, T2 arg2);

type Func<'T1, 'T2, 'Result> = delegate of 'T1 * 'T2 -> 'Result

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

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

Type Parameters

Parameters

Return Value

The return value of the method that this delegate encapsulates.

Examples

The following example demonstrates how to declare and use a Func delegate. This example declares a Func 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.

using System;
using System.Collections.Generic;
using System.Linq;

public class Func3Example
{
   public static void Main()
   {
      Func predicate = (str, index) => str.Length == index;

      String[] words = { "orange", "apple", "Article", "elephant", "star", "and" };
      IEnumerable aWords = words.Where(predicate).Select(str => str);

      foreach (String word in aWords)
         Console.WriteLine(word);
   }
}

open System
open System.Linq

let predicate = Func(fun str index -> str.Length = index)

let words = [ "orange"; "apple"; "Article"; "elephant"; "star"; "and" ]
let aWords = words.Where predicate

for word in aWords do
    printfn $"{word}"

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

Remarks

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.

When you use the Func 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.

using System;

delegate string[] ExtractMethod(string stringToManipulate, int maximum);

public class DelegateExample
{
   public static void Main()
   {
      // Instantiate delegate to reference ExtractWords method
      ExtractMethod extractMeth = ExtractWords;
      string title = "The Scarlet Letter";
      // Use delegate instance to call ExtractWords method and display result
      foreach (string word in extractMeth(title, 5))
         Console.WriteLine(word);
   }

   private static string[] ExtractWords(string phrase, int limit)
   {
      char[] delimiters = new char[] {' '};
      if (limit > 0)
         return phrase.Split(delimiters, limit);
      else
         return phrase.Split(delimiters);
   }
}

type ExtractMethod = delegate of string * int -> string []

let extractWords (phrase: string) limit =
    let delimiters = [| ' ' |]
    if limit > 0 then
        phrase.Split(delimiters, limit)
    else
        phrase.Split delimiters

// Instantiate delegate to reference extractWords function
let extractMeth = ExtractMethod extractWords
let title = "The Scarlet Letter"

// Use delegate instance to call extractWords function and display result
for word in extractMeth.Invoke(title, 5) do
    printfn $"{word}"

' 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 delegate instead of explicitly defining a new delegate and assigning a named method to it.

using System;

public class GenericFunc
{
   public static void Main()
   {
      // Instantiate delegate to reference ExtractWords method
      Func extractMethod = ExtractWords;
      string title = "The Scarlet Letter";
      // Use delegate instance to call ExtractWords method and display result
      foreach (string word in extractMethod(title, 5))
         Console.WriteLine(word);
   }

   private static string[] ExtractWords(string phrase, int limit)
   {
      char[] delimiters = new char[] {' '};
      if (limit > 0)
         return phrase.Split(delimiters, limit);
      else
         return phrase.Split(delimiters);
   }
}

open System

let extractWords (phrase: string) limit =
    let delimiters = [| ' ' |]
    if limit > 0 then
        phrase.Split(delimiters, limit)
    else
        phrase.Split delimiters

// Instantiate delegate to reference extractWords function
let extractMethod = Func extractWords
let title = "The Scarlet Letter"

// Use delegate instance to call extractWords function and display result
for word in extractMethod.Invoke(title, 5) do
    printfn $"{word}"

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 delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods.)

using System;

public class Anonymous
{
   public static void Main()
   {
      Func 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 delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (VB), Lambda Expressions (C#) and Lambda Expressions (F#).)

using System;

public class LambdaExpression
{
   public static void Main()
   {
      char[] separators = new char[] {' '};
      Func extract = (s, i) =>
           i > 0 ? s.Split(separators, i) : s.Split(separators) ;

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

open System

let separators = [| ' ' |]

let extract =
    Func (fun s i ->
        if i > 0 then
            s.Split(separators, i)
        else
            s.Split separators)

let title = "The Scarlet Letter"

// Use Func instance to call lambda expression and display result
for word in extract.Invoke(title, 5) do
    printfn $"{word}"

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 parameters, you can pass these methods a lambda expression without explicitly instantiating a Func delegate.

See also

• Lambda Expressions (C# Programming Guide)
• Lambda Expressions: The fun Keyword (F#)
• Lambda Expressions
• Delegates (C# Programming Guide)
• Delegates (F#)
• Delegates in Visual Basic