C# 閉包問題-你被」坑「過嗎？

時間 2019-11-06

標籤 c# 閉包問題欄目 C# 简体版

原文原文鏈接

引言

閉包是什麼？之前看面試題的時候才發現這個名詞。html

閉包在實際項目中會有什麼問題？如今就讓咱們一塊兒來看下這個不太熟悉的名詞。面試

若是在實際工做中用到了匿名函數和lamada表達式,那你就應該高度注意啦.編程

問題

請問下你們這段代碼的輸出結果是什麼樣的呢？閉包

public static void Main()
{
    Console.WriteLine("Starting.");

    for (int i = 0; i < 4; ++i)
        Task.Run(() => Console.WriteLine(i));

    Console.WriteLine("Finished. Press <ENTER> to exit.");
    Console.ReadLine();
}

輸出結果:ide

Starting.
Finished. Press <ENTER> to exit.
4
4
4
4

你答對了嗎？若是沒有請跟隨我一塊兒來看下這裏的深層緣由。函數

問題解決

public static void Main()
{
    Console.WriteLine("Starting.");

    for (int i = 0; i < 4; ++i)
    {
        int j = i;
        Task.Run(() => Console.WriteLine(j));
    }

    Console.WriteLine("Finished. Press <ENTER> to exit.");
    Console.ReadLine();
}

輸出結果oop

Starting.
Finished. Press <ENTER> to exit.
0
1
3
2

緣由分析

閉包是什麼？

using System;

class Test
{
    static void Main()
    {
        Action action = CreateAction();
        action();
        action();
    }

    static Action CreateAction()
    {
        int counter = 0;
        return delegate
        {
            // Yes, it could be done in one statement; 
            // but it is clearer like this.
            counter++;
            Console.WriteLine("counter={0}", counter);
        };
    }
}

輸出this

counter=1
counter=2

In essence, a closure is a block of code which can be executed at a later time, but which maintains the environment in which it was first created - i.e. it can still use the local variables etc of the method which created it, even after that method has finished executing.spa

這段話的大意是：從本質上說，閉包是一段能夠在晚些時候執行的代碼塊，可是這段代碼塊依然維護着它第一個被建立時環境（執行上下文）- 即它仍可使用建立它的方法中局部變量，即便那個方法已經執行完了。code

這段話準確地來講不能算做定義，但形象的給出了描述。這裏就不給出絕對定義啦。wiki上有這方面的描述。

C#中一般經過匿名函數和lamada表達式來實現閉包。

再來看一個簡單的例子：

var values = new List<int> { 100, 110, 120 };
var funcs = new List<Func<int>>();

foreach (var v in values)
    funcs.Add(() =>
    {
        //Console.WriteLine(v);
        return v;
    });

foreach (var f in funcs)
    Console.WriteLine(f());

Console.WriteLine("{0}{0}", Environment.NewLine);


funcs.Clear();
for (var i = 0; i < values.Count; i++)
{

 //var v2 = values[i];

   funcs.Add(() =>
    {
        
       var v2 = values[i]; //will throw exception

        return v2;
    });
}

foreach (var f in funcs)
    Console.WriteLine(f());

一語道破天機

Because ()=>v means "return the current value of variable v", not "return the value v was back when the delegate was created"，Closures close over variables, not over values.

原文大意：由於() = > v "返回變量 v 的當前值"，而不是建立該委託時"v「的返回值。閉包」變量「，而不是閉包」值「。

因此在」for「循環中的添加的匿名函數，只是返回了變量i 而不是i的值。因此知道f() 被真正執行時，i已是values.Count 值啦，因此會拋出」超出索引範圍「。那爲啥foreach 沒事呢？那就讓咱們接着看下閉包的來頭。

歷史簡述

The latter. The C# 1.0 specification actually did not say whether the loop variable was inside or outside the loop body, as it made no observable difference. When closure semantics were introduced in C# 2.0, the choice was made to put the loop variable outside the loop, consistent with the "for" loop.--Eric Lippert

閉包在C#2.0 的時候引入了閉包語法，選擇將循環變量放在循環體外面，for 和foreach 在這方面處理都是一致的。但隨着人們在使用過程當中的種種不適，微軟作出了」一點「讓步，在C#5 中對」foreach「作了調整，但對」for「沒有作改動。具體改動以下說：

We are taking the breaking change. In C# 5, the loop variable of a foreach will be logically inside the loop, and therefore closures will close over a fresh copy of the variable each time. The "for" loop will not be changed. --Eric Lippert

原文大意：在C#5中咱們作了巨大的調整，「foreach」的遍歷中的定義的臨時循環變量會被邏輯上限制在循環內，「foreach」的每次循環都會是循環變量的一個拷貝，這樣閉包就看起來關閉了（沒有了）。但「for」循環沒有作修改。