Func<T,TResult> 委托

定义

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<in T,out TResult>(T arg) where T : allows ref struct where TResult : allows ref struct;

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 

类型参数

参数

返回值

此委托封装的方法的返回值。

示例

以下示例演示如何声明和使用 Func<T,TResult> 委托。 此示例声明一个 Func<T,TResult> 变量，并为其分配一个 lambda 表达式，该表达式将字符串中的字符转换为大写。 随后将封装此方法的委托传递给 Enumerable.Select 方法，以将字符串数组中的字符串更改为大写。

// 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

注解

可以使用此委托来表示可以作为参数传递的方法，而无需显式声明自定义委托。 封装的方法必须与此委托定义的方法签名相对应。 这意味着封装的方法必须有一个参数，该参数通过值传递给它，并且必须返回一个值。

使用 Func<T,TResult> 委托时，无需显式定义封装具有单个参数的方法的委托。 例如，以下代码显式声明命名 ConvertMethod 的委托，并将对方法的 UppercaseString 引用分配给其委托实例。

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

以下示例通过实例化 Func<T,TResult> 委托而不是显式定义新委托并向其分配命名方法来简化此代码。

// 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

还可以将 Func<T,TResult> 委托与 C# 中的匿名方法一起使用，如以下示例所示。 （有关匿名方法的简介，请参阅 匿名方法。

 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

还可以将 lambda 表达式分配给 Func<T,TResult> 委托，如以下示例所示。 （有关 lambda 表达式的简介，请参阅 Lambda 表达式（VB）、Lambda 表达式（C#）和 Lambda 表达式（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

lambda 表达式的基础类型是泛型 Func 委托之一。 这样就可以将 lambda 表达式作为参数传递，而无需将其显式分配给委托。 具体而言，由于命名空间中的 System.Linq 许多类型方法都有 Func<T,TResult> 参数，因此你可以将这些方法作为 lambda 表达式传递，而无需显式实例化 Func<T,TResult> 委托。

另请参阅

• Lambda 表达式 （C# 编程指南）
• Lambda 表达式：有趣的关键字 （F#）
• Lambda 表达式（Visual Basic）
• 委托（C# 编程指南）
• 委托（F#）
• Delegates （Visual Basic）