Func<T,TResult> Delegate

Definition

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

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

The parameter of the method that this delegate encapsulates.

Return Value
TResult

The return value of the method that this delegate encapsulates.

Inheritance
Func<T,TResult>

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.

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

static class Func
{
   static void Main(string[] args)
   {
      // 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
*/
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

Remarks

Note

To view the .NET Framework source code for this type, see the Reference Source. You can browse through the source code online, download the reference for offline viewing, and step through the sources (including patches and updates) during debugging; see instructions.

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();
   }
}
' 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.

using System;

public class GenericFunc
{
   public static void Main()
   {
      // 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));
   }

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

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<T,TResult> delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions.)

using System;

public class LambdaExpression
{
   public static void Main()
   {
      Func<string, string> convert = s => s.ToUpper();
         
      string name = "Dakota";
      Console.WriteLine(convert(name));   
   }
}
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.