Func<T,TResult> 委托

定义

封装一个具有一个参数并返回 TResult 参数指定的类型值的方法。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 

类型参数

T

此委托封装的方法的参数类型。The type of the parameter of the method that this delegate encapsulates.

这是逆变类型参数。 即,可以使用指定的类型,也可以使用派生程度较低的任何类型。 有关协变和逆变的详细信息,请参阅泛型中的协变和逆变
TResult

此委托封装的方法的返回值类型。The type of the return value of the method that this delegate encapsulates.

这是协变类型参数。 即,可以使用指定的类型,也可以使用派生程度较高的任何类型。 有关协变和逆变的详细信息,请参阅泛型中的协变和逆变

参数

arg
T

此委托封装的方法的参数。The parameter of the method that this delegate encapsulates.

返回值

TResult

此委托封装的方法的返回值。The return value of the method that this delegate encapsulates.

继承
Func<T,TResult>

示例

下面的示例演示如何声明和使用 Func<T,TResult> 委托。The following example demonstrates how to declare and use a Func<T,TResult> delegate. 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。This example declares a Func<T,TResult> variable and assigns it a lambda expression that converts the characters in a string to uppercase. 然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。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

*/
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

您可以使用此委托来表示一个方法,该方法可作为参数传递,而无需显式声明自定义委托。You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. 封装的方法必须对应于由此委托定义的方法签名。The encapsulated method must correspond to the method signature that is defined by this delegate. 这意味着,封装的方法必须具有一个通过值传递给它的参数,并且必须返回一个值。This means that the encapsulated method must have one parameter that is passed to it by value, and that it must return a value.

备注

若要引用一个方法,该方法具有一个参数并返回 void (或者在 Visual Basic 中,该方法被声明为 Sub 而不是 Function),请改用泛型 Action<T> 委托。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.

使用 Func<T,TResult> 委托时,无需显式定义使用单个参数封装方法的委托。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. 例如,下面的代码显式声明名为 ConvertMethod 的委托,并向其委托实例分配对 UppercaseString 方法的引用。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

下面的示例通过实例化 Func<T,TResult> 委托简化了此代码,而不是显式定义一个新委托并为其分配一个命名方法。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
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

你还可以在中C#使用匿名方法的 Func<T,TResult> 委托,如下面的示例所示。You can also use the Func<T,TResult> delegate with anonymous methods in C#, as the following example illustrates. (有关匿名方法的介绍,请参阅匿名方法。)(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

你还可以将 lambda 表达式分配给 Func<T,TResult> 委托,如下面的示例所示。You can also assign a lambda expression to a Func<T,TResult> delegate, as the following example illustrates. (有关 lambda 表达式的介绍,请参阅Lambda 表达式lambda 表达式。)(For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions.)

Func<string, string> convert = s => s.ToUpper();
   
string name = "Dakota";
Console.WriteLine(convert(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。The underlying type of a lambda expression is one of the generic Func delegates. 这样,便可以将 lambda 表达式作为参数传递,而无需将其显式分配给委托。This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate. 具体来说,因为 System.Linq 命名空间中的许多类型的方法都具有 Func<T,TResult> 参数,所以可以将这些方法传递给 lambda 表达式,而无需显式实例化 Func<T,TResult> 委托。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.

扩展方法

GetMethodInfo(Delegate)

获取指示指定委托表示的方法的对象。Gets an object that represents the method represented by the specified delegate.

适用于

另请参阅