Func<T1,T2,TResult> Delegate
Definition
Important
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Encapsulates a method that has two parameters and returns a value of the type specified by the TResult parameter.
generic <typename T1, typename T2, typename TResult>
public delegate TResult Func(T1 arg1, T2 arg2);public delegate TResult Func<in T1,in T2,out TResult>(T1 arg1, T2 arg2);public delegate TResult Func<T1,T2,TResult>(T1 arg1, T2 arg2);type Func<'T1, 'T2, 'Result> = delegate of 'T1 * 'T2 -> 'ResultPublic 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
- 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.- T2
The type of the second 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.- 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
- T1
The first parameter of the method that this delegate encapsulates.
- arg2
- T2
The second parameter of the method that this delegate encapsulates.
Return Value
The return value of the method that this delegate encapsulates.
Examples
The following example demonstrates how to declare and use a Func<T1,T2,TResult> delegate. This example declares a Func<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.
using System;
using System.Collections.Generic;
using System.Linq;
public class Func3Example
{
public static void Main()
{
Func<String, int, bool> predicate = (str, index) => str.Length == index;
String[] words = { "orange", "apple", "Article", "elephant", "star", "and" };
IEnumerable<String> aWords = words.Where(predicate).Select(str => str);
foreach (String word in aWords)
Console.WriteLine(word);
}
}
open System
open System.Linq
let predicate = Func<string, int, bool>(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.
Note
To reference a method that has two parameters and returns void (unit in F#) (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action<T1,T2> delegate instead.
When you use the Func<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.
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<T1,T2,TResult> 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<string, int, string[]> 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<string, int, string[]> 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<T1,T2,TResult> 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<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<T1,T2,TResult> 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<string, int, string[]> 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<string, int, string []> (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<T1,T2,TResult> parameters, you can pass these methods a lambda expression without explicitly instantiating a Func<T1,T2,TResult> delegate.
Extension Methods
| GetMethodInfo(Delegate) |
Gets an object that represents the method represented by the specified delegate. |
