问题描述

我一直试图了解 C# 中的委托，但我似乎不明白使用它们的意义.这是来自 MSDN 代表页面:

I have been trying to get my head around delegates in C#, but I just don't seem to get the point of using them. Here is some slightly reconstructed code from the MSDN page on delegates:

using System;
using System.Collections;

namespace Delegates
{
    // Describes a book in the book list:
    public struct Book
    {
        public string Title;        // Title of the book.
        public string Author;       // Author of the book.
        public decimal Price;       // Price of the book.
        public bool Paperback;      // Is it paperback?

        public Book(string title, string author, decimal price, bool paperBack)
        {
            Title = title;
            Author = author;
            Price = price;
            Paperback = paperBack;
        }
    }

    // Declare a delegate type for processing a book:
    public delegate void ProcessBookDelegate(Book book);

    // Maintains a book database.
    public class BookDB
    {
        // List of all books in the database:
        ArrayList list = new ArrayList();

        // Add a book to the database:
        public void AddBook(string title, string author, decimal price, bool paperBack)
        {
            list.Add(new Book(title, author, price, paperBack));
        }

        // Call a passed-in delegate on each paperback book to process it:
        public void ProcessPaperbackBooksWithDelegate(ProcessBookDelegate processBook)
        {
            foreach (Book b in list)
            {
                if (b.Paperback)
                    processBook(b);
            }
        }

        public void ProcessPaperbackBooksWithoutDelegate(Action<Book> action)
        {
            foreach (Book b in list)
            {
                if (b.Paperback)
                    action(b);
            }
        }
    }

    class Test
    {

        // Print the title of the book.
        static void PrintTitle(Book b)
        {
            Console.WriteLine("   {0}", b.Title);
        }

        // Execution starts here.
        static void Main()
        {
            BookDB bookDB = new BookDB();
            AddBooks(bookDB);
            Console.WriteLine("Paperback Book Titles Using Delegates:");
            bookDB.ProcessPaperbackBooksWithDelegate(new ProcessBookDelegate(PrintTitle));
            Console.WriteLine("Paperback Book Titles Without Delegates:");
            bookDB.ProcessPaperbackBooksWithoutDelegate(PrintTitle);
        }

        // Initialize the book database with some test books:
        static void AddBooks(BookDB bookDB)
        {
            bookDB.AddBook("The C Programming Language",
               "Brian W. Kernighan and Dennis M. Ritchie", 19.95m, true);
            bookDB.AddBook("The Unicode Standard 2.0",
               "The Unicode Consortium", 39.95m, true);
            bookDB.AddBook("The MS-DOS Encyclopedia",
               "Ray Duncan", 129.95m, false);
            bookDB.AddBook("Dogbert's Clues for the Clueless",
               "Scott Adams", 12.00m, true);
        }
    }
}

正如您在 BookDB 类中看到的，我定义了 2 种不同的方法:

As you can see in the BookDB class, I have defined 2 different methods:

1. 以委托作为参数的方法:ProcessPaperbackBooksWithDelegate
2. 一个将相应类型签名的动作作为参数的:ProcessPaperbackBooksWithoutDelegate

调用它们中的任何一个都会返回相同的结果；那么委托解决的目的是什么?

A call to either of them returns the same result; so what purpose does a delegate solve?

同一页面上的第二个示例会导致更多混乱；这是代码:

The second example on the same page leads to lot more confusion; here is the code:

delegate void MyDelegate(string s);

static class MyClass
{
    public static void Hello(string s)
    {
        Console.WriteLine("  Hello, {0}!", s);
    }

    public static void Goodbye(string s)
    {
        Console.WriteLine("  Goodbye, {0}!", s);
    }

    public static string HelloS(string s)
    {
        return string.Format("Hello, {0}!", s);
    }

    public static string GoodbyeS(string s)
    {
        return string.Format("Goodbye, {0}!", s);
    }

    public static void Main1()
    {
        MyDelegate a, b, c, d;
        a = new MyDelegate(Hello);
        b = new MyDelegate(Goodbye);
        c = a + b;
        d = c - a;

        Console.WriteLine("Invoking delegate a:");
        a("A");
        Console.WriteLine("Invoking delegate b:");
        b("B");
        Console.WriteLine("Invoking delegate c:");
        c("C");
        Console.WriteLine("Invoking delegate d:");
        d("D");
    }

    public static void Main2()
    {
        Action<string> a = Hello;
        Action<string> b = Goodbye;
        Action<string> c = a + b;
        Action<string> d = c - a;

        Console.WriteLine("Invoking delegate a:");
        a("A");
        Console.WriteLine("Invoking delegate b:");
        b("B");
        Console.WriteLine("Invoking delegate c:");
        c("C");
        Console.WriteLine("Invoking delegate d:");
        d("D");
    }

    public static void Main3()
    {
        Func<string, string> a = HelloS;
        Func<string, string> b = GoodbyeS;
        Func<string, string> c = a + b;
        Func<string, string> d = c - a;

        Console.WriteLine("Invoking function a: " + a("A"));
        Console.WriteLine("Invoking function b: " + b("B"));
        Console.WriteLine("Invoking function c: " + c("C"));
        Console.WriteLine("Invoking function d: " + d("D"));
    }
}

Main1 是示例中已有的函数.Main2 和 Main3 是我添加的小提琴.

Main1 is the function which was already in the example. Main2 and Main3 are fiddles added by me.

正如我所料，Main1 和 Main2 给出相同的结果，即:

As I expected, Main1 and Main2 give the same result i.e.:

Invoking delegate a:
  Hello, A!
Invoking delegate b:
  Goodbye, B!
Invoking delegate c:
  Hello, C!
  Goodbye, C!
Invoking delegate d:
  Goodbye, D!

Main3 然而，给出了一个非常奇怪的结果:

Main3 however, gives a very strange result:

Invoking function a: Hello, A!
Invoking function b: Goodbye, B!
Invoking function c: Goodbye, C!
Invoking function d: Goodbye, D!

如果 + 实际上是在执行函数组合，那么结果(对于 Main3)应该是:

If + was actually performing function composition then the result(for Main3) should have been:

Invoking function a: Hello, A!
Invoking function b: Goodbye, B!
Invoking function c: Hello, Goodbye, C!!
Invoking function d: //God knows what this should have been.

但很明显，+ 实际上并不是传统的函数式组合(我猜，真正的组合甚至不适用于 Action).从它似乎没有类型签名的事实可以看出这一点:

But it is clear that + isn't actually the traditional functional composition(real composition wouldn't even work for an Action, I guess). That much is evident from the fact that it doesn't seem to have a type signature of:

(T2 -> T3) -> (T1 -> T2) -> T1 -> T3

相反，类型签名似乎是:

instead, the type signature seems to be:

(T1 -> T2) -> (T1 -> T2) -> (T1 -> T2)

那么+和-到底是什么意思?

So what do + and - really mean?

旁白:我尝试在 Main2 中使用 var a = Hello;... 但得到错误:

Aside: I tried to use var a = Hello;... in Main2 but got the error:

test.cs(136,14): error CS0815: Cannot assign method group to an implicitly-typed
    local variable

可能与这个问题无关，但为什么不能这样做呢?这似乎是一个非常直接的类型推断.

It may not be related to this question, but why can't it do so? It seems like a pretty straight forward type deduction.

推荐答案

自定义委托类型 vs Func 和 Action

当您可以通过 delegate 获得相同的结果时，为什么还要使用 Func 和/或 Action?

因为:

Custom delegate types vs Func and Action

Why use Func and/or Action when you can achieve the same results with a delegate?

Because:

• 它省去了为每个可能的方法签名创建自定义委托类型的麻烦.在代码中，少即是多.
• 不同的自定义委托类型是不兼容的，即使它们的签名完全匹配.您可以解决这个问题，但它很冗长.
• 自从引入 Func 和 Action 以来，这是编写代码的惯用方式.除非有相反的令人信服的理由，否则你想像罗马人那样做.

• It saves you the trouble of creating a custom delegate type for each possible method signature. In code, less is more.
• Different custom delegate types are incompatible, even if their signatures exactly match. You can work around this but it's verbose.
• Since the introduction of Func and Action this is the idiomatic way to write code. Unless there is compelling reason for the opposite, you want to do as the Romans do.

让我们看看问题是什么:

Let's see what the problem is:

// Delegates: same signature but different types
public delegate void Foo();
public delegate void Bar();

// Consumer function -- note it accepts a Foo
public void Consumer(Foo f) {}

试一试:

Consumer(new Foo(delegate() {})); // works fine
Consumer(new Bar(delegate() {})); // error: cannot convert "Bar" to "Foo"

最后一行是有问题的:没有技术原因导致它无法工作，但编译器将 Foo 和 Bar 视为不同的类型并禁止它.这可能会导致摩擦，因为如果您只有一个 Bar 您将不得不编写

The last line is problematic: there is no technical reason why it cannot work, but the compiler treats Foo and Bar as the distinct types they are and disallows it. This can lead to friction because if all you have is a Bar you would have to write

var bar = new Bar(delegate() {});
Consumer(new Foo(bar)); // OK, but the ritual isn't a positive experience

为什么在 Func 和/或 Action 上使用委托?

因为:

Why use a delegate over Func and/or Action?

Because:

• 您的目标是早期版本的 C#，其中不存在这些类型.
• 您正在处理复杂的函数签名.没有人会想要多次输入:Func.

void Foo() {}
void Bar() {}

var a = new Action(Foo) + Bar;
a(); // calls both Foo() and Bar()

var a = new Action(Foo);
a();      // calls Foo()
a -= Bar; // Bar is not a part of the multicast delegate; nothing happens
a();      // still calls Foo() as before

public int Ret1() { return 1; }
public int Ret2() { return 2; }

Console.WriteLine((new Func<int>(Ret1) + Ret2)()); // prints "2"
Console.WriteLine((new Func<int>(Ret2) + Ret1)()); // prints "1"

public bool IsInteresting(int i) { return i != 0; }
public bool IsInteresting(string s) { return s != ""; }

// both of these convert the method group to the "obviously correct" delegate
Func<int, bool> f1 = IsInteresting;
Func<string, bool> f2 = IsInteresting;