Func<T,TResult> Delegate

Reference

Definition

Namespace:: System

Assembly:: System.Runtime.dll

Assembly:: System.Core.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Important

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

generic <typename T, typename TResult>
public delegate TResult Func(T arg);

public delegate TResult Func<in T,out TResult>(T arg);

public delegate TResult Func<T,TResult>(T arg);

type Func<'T, 'Result> = delegate of 'T -> 'Result

Public Delegate Function Func(Of In T, Out TResult)(arg As T) As TResult

Public Delegate Function Func(Of T, TResult)(arg As T) As TResult

Type Parameters

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 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

arg: T

The parameter of the method that this delegate encapsulates.

Return Value

TResult

The return value of the method that this delegate encapsulates.

Examples

The following example demonstrates how to declare and use a Func<T,TResult> delegate. This example declares a Func<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 Enumerable.Select method to change the strings in an array of strings to uppercase.

// Declare a Func variable and assign a lambda expression to the
// variable. The method takes a string and converts it to uppercase.
Func<string, string> selector = str => str.ToUpper();

// Create an array of strings.
string[] words = { "orange", "apple", "Article", "elephant" };
// Query the array and select strings according to the selector method.
IEnumerable<String> aWords = words.Select(selector);

// Output the results to the console.
foreach (String word in aWords)
    Console.WriteLine(word);

/*
This code example produces the following output:

ORANGE
APPLE
ARTICLE
ELEPHANT

*/

open System
open System.Linq

// Declare a Func variable and assign a lambda expression to the
// variable. The function takes a string and converts it to uppercase.
let selector = Func<string, string>(fun str -> str.ToUpper())

// Create a list of strings.
let words = [ "orange"; "apple"; "Article"; "elephant" ]

// Query the list and select strings according to the selector function.
let aWords = words.Select selector

// Output the results to the console.
for word in aWords do
    printfn $"{word}"

// This code example produces the following output:
//     ORANGE
//     APPLE
//     ARTICLE
//     ELEPHANT

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

// Declare a delegate
delegate String ^ MyDel(String ^);

// Create wrapper class and function that takes in a string and converts it to uppercase
ref class DelegateWrapper {
public:
    String ^ ToUpper(String ^ s) {
        return s->ToUpper();
    }
};

int main() {
    // Declare delegate
    DelegateWrapper ^ delegateWrapper = gcnew DelegateWrapper;
    MyDel ^ DelInst = gcnew MyDel(delegateWrapper, &DelegateWrapper::ToUpper);

    // Cast into Func
    Func<String ^, String ^> ^ selector = reinterpret_cast<Func<String ^, String ^> ^>(DelInst);

    // Create an array of strings
    array<String ^> ^ words = { "orange", "apple", "Article", "elephant" };

    // Query the array and select strings according to the selector method
    Generic::IEnumerable<String ^> ^ aWords = Enumerable::Select((Generic::IEnumerable<String^>^)words, selector);

    // Output the results to the console
    for each(String ^ word in aWords)
        Console::WriteLine(word);
    /*
    This code example produces the following output:

    ORANGE
    APPLE
    ARTICLE
    ELEPHANT
    */
}

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 one parameter that is passed to it by value, and that it must return a value.

Note

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<T> delegate instead.

When you use the Func<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.

using System;

delegate string ConvertMethod(string inString);

public class DelegateExample
{
   public static void Main()
   {
      // Instantiate delegate to reference UppercaseString method
      ConvertMethod convertMeth = UppercaseString;
      string name = "Dakota";
      // Use delegate instance to call UppercaseString method
      Console.WriteLine(convertMeth(name));
   }

   private static string UppercaseString(string inputString)
   {
      return inputString.ToUpper();
   }
}

type ConvertMethod = delegate of string -> string

let uppercaseString (inputString: string) =
    inputString.ToUpper()
    
// Instantiate delegate to reference uppercaseString function
let convertMeth = ConvertMethod uppercaseString
let name = "Dakota"

// Use delegate instance to call uppercaseString function
printfn $"{convertMeth.Invoke name}"

' 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<T,TResult> delegate instead of explicitly defining a new delegate and assigning a named method to it.

// Instantiate delegate to reference UppercaseString method
Func<string, string> convertMethod = UppercaseString;
string name = "Dakota";
// Use delegate instance to call UppercaseString method
Console.WriteLine(convertMethod(name));

string UppercaseString(string inputString)
{
   return inputString.ToUpper();
}

// This code example produces the following output:
//
//    DAKOTA

open System

let uppercaseString (inputString: string) =
    inputString.ToUpper()

// Instantiate delegate to reference uppercaseString function
let convertMethod = Func<string, string> uppercaseString
let name = "Dakota"

// Use delegate instance to call uppercaseString function
printfn $"{convertMethod.Invoke name}"

// This code example produces the following output:
//    DAKOTA

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

 Func<string, string> convert = delegate(string s)
    { return s.ToUpper();};

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

// This code example produces the following output:
//
//    DAKOTA

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

Func<string, string> convert = s => s.ToUpper();

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

// This code example produces the following output:
//
//    DAKOTA

open System

let convert = Func<string, string>(fun s -> s.ToUpper())

let name = "Dakota"
printfn $"{convert.Invoke name}"

// This code example produces the following output:
//    DAKOTA

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

Extension Methods

GetMethodInfo(Delegate)

Gets an object that represents the method represented by the specified delegate.

Applies to

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