問題描述

考慮以下代碼:

公共類 MyClass{公共委托字符串 PrintHelloType(字符串問候語)；公共無效執(zhí)行(){Type[] types = new Type[] { typeof(string), typeof(float), typeof(int)};列表<PrintHelloType>helloMethods = 新列表<PrintHelloType>();foreach(類型中的 var 類型){var sayHello =new PrintHelloType(greeting => SayGreetingToType(type, greeting));helloMethods.Add(sayHello);}foreach(helloMethods 中的 var helloMethod){Console.WriteLine(helloMethod("Hi"));}}public string SayGreetingToType(Type type, string greetingText){返回 greetingText + " " + type.Name;}...}

調(diào)用myClass.Execute()后，代碼打印出以下意外響應(yīng):

顯然，我希望 "Hi String"、"Hi Single"、"Hi Int32"，但顯然不是案件.為什么在所有 3 種方法中都使用迭代數(shù)組的最后一個元素而不是適當(dāng)?shù)姆椒?

您將如何重寫代碼以實(shí)現(xiàn)預(yù)期目標(biāo)?

歡迎來到閉包和捕獲變量的世界:)

Eric Lippert 對此行為有深入的解釋:

• 結(jié)束在被認(rèn)為有害的循環(huán)變量上
• 結(jié)束在循環(huán)變量上，第二部分

基本上，捕獲的是循環(huán)變量，而不是值.要獲得您認(rèn)為應(yīng)該獲得的東西，請執(zhí)行以下操作:

foreach (var type in types){var newType = 類型；var sayHello =new PrintHelloType(greeting => SayGreetingToType(newType, greeting));helloMethods.Add(sayHello);}

Consider the following code:

public class MyClass
{
   public delegate string PrintHelloType(string greeting);


    public void Execute()
    {

        Type[] types = new Type[] { typeof(string), typeof(float), typeof(int)};
        List<PrintHelloType> helloMethods = new List<PrintHelloType>();

        foreach (var type in types)
        {
            var sayHello = 
                new PrintHelloType(greeting => SayGreetingToType(type, greeting));
            helloMethods.Add(sayHello);
        }

        foreach (var helloMethod in helloMethods)
        {
            Console.WriteLine(helloMethod("Hi"));
        }

    }

    public string SayGreetingToType(Type type, string greetingText)
    {
        return greetingText + " " + type.Name;
    }

...

}

After calling myClass.Execute(), the code prints the following unexpected response:

Hi Int32
Hi Int32
Hi Int32  

Obviously, I would expect "Hi String", "Hi Single", "Hi Int32", but apparently it is not the case. Why the last element of the iterated array is being used in all the 3 methods instead of the appropriate one?

How would you rewrite the code to achieve the desired goal?

Welcome to the world of closures and captured variables :)

Eric Lippert has an in-depth explanation of this behaviour:

• Closing over the loop variable considered harmful
• Closing over the loop variable, part two

basically, it's the loop variable that is captured, not it's value. To get what you think you should get, do this:

foreach (var type in types)
{
   var newType = type;
   var sayHello = 
            new PrintHelloType(greeting => SayGreetingToType(newType, greeting));
   helloMethods.Add(sayHello);
}

這篇關(guān)于在 lambda 表達(dá)式中使用 foreach 循環(huán)的迭代器變量 - 為什么會失敗?的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！