冻结瓶盖的优雅方法 [英] Elegant way to freeze closures

问题描述

是否有任何优雅的方法来冻结"从方法返回的操作中使用的变量?

Is there any elegant way to "freeze" the variables used in an action that is returned from a method?

只需看下面的代码:

static void Main(String[] args) 
{
    foreach(Action a in ClosureTrap("a", "b")) 
    {
        a();
    }
}

static List<Action> ClosureTrap(params String[] strings) 
{
    List<Action> result = new List<Action>();
    foreach(String s in strings) 
    {
        result.Add(() => Console.WriteLine(s));
    }
    return result;
}

此代码将向控制台写入两行，均包含"b".这样做的原因并不难找到:ClosureTrap中"s"的最后一个值为"b".

This code will write two lines to the console, both containing "b". The reason for this is not too hard to find: The last value for "s" in ClosureTrap is "b".

是否有任何优雅的方法来获得两行"a"和"b"作为控制台上的输出?

Is there any elegant way to get two lines "a" and "b" as output on the console?

当前，我正在使用其他方法来创建委托.但是通过这样做，这些封盖就失去了许多优雅之处:

Currently I am using an additional method to create the delegate. But by doing this, the closures loose a lot of their elegance:

static List<Action> ClosureTrap(params String[] strings) 
{
    List<Action> result = new List<Action>();
    foreach(String s in strings) 
    {
        result.Add(Freeze(s));
    }
    return result;
}

static Action Freeze(String s) 
{
    return () => Console.WriteLine(s);
}

有更好的方法吗?

推荐答案

没有通用的 方法-但是，如果您仅受 specific 问题困扰的 foreach ，则有两个选项:

There's no general way to do this - but if you're only bothered by the specific issue of foreach, then there are two options:

• 开始使用C#5编译器(您不需要定位.NET 4.5或类似的东西).关于捕获的 foreach的规则 C#5的迭代变量更改为(健全).

• 如果您使用C#3或4，请捕获在循环中声明的局部变量 :

• Start using a C# 5 compiler (you don't need to target .NET 4.5 or anything like that). The rules around captured foreach iteration variables changed (to sane ones) for C# 5.

• If you're stuck with C# 3 or 4, capture a local variable declared in the loop:

foreach(String s in strings) 
{
    string copy = s;
    result.Add(() => Console.WriteLine(copy));
} 

这样，循环的每次迭代都会捕获一个单独的变量，该变量永远不会更改值.

That way each iteration of the loop captures a separate variable, which never changes value.

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