问题描述
这是问题的扩展。我只想验证以下是否真的足够安全用于生产使用。
This is an extension of question from Access to Modified Closure. I just want to verify if the following is actually safe enough for production use.
List<string> lists = new List<string>();
//Code to retrieve lists from DB
foreach (string list in lists)
{
Button btn = new Button();
btn.Click += new EventHandler(delegate { MessageBox.Show(list); });
}
我每次启动只运行一次。现在它似乎工作好了。正如Jon在某些情况下提到了违反直觉的结果。那么我需要在这里注意什么？
I only run through the above once per startup. For now it seems to work alright. As Jon has mentioned about counterintuitive result in some case. So what do I need to watch out here? Will it be ok if the list is run through more than once?
推荐答案
在C＃5之前，您需要重新声明在foreach中的变量 - 否则它是共享的，并且所有的处理程序将使用最后一个字符串：
Prior to C# 5, you need to re-declare a variable inside the foreach - otherwise it is shared, and all your handlers will use the last string:
foreach (string list in lists)
{
string tmp = list;
Button btn = new Button();
btn.Click += new EventHandler(delegate { MessageBox.Show(tmp); });
}
值得注意的是，从C＃5开始， >特别是在 foreach 的情况下，您不需要这样做：问题中的代码将按预期工作。
Significantly, note that from C# 5 onwards, this has changed, and specifically in the case of foreach, you do not need to do this any more: the code in the question would work as expected.
如果没有此更改，则显示此选项无效。
To show this not working without this change, consider the following:
string[] names = { "Fred", "Barney", "Betty", "Wilma" };
using (Form form = new Form())
{
foreach (string name in names)
{
Button btn = new Button();
btn.Text = name;
btn.Click += delegate
{
MessageBox.Show(form, name);
};
btn.Dock = DockStyle.Top;
form.Controls.Add(btn);
}
Application.Run(form);
}
运行上述之前的C＃5
这是因为语言规格（ECMA 334 v4,15.8.4）（之前是C＃5）定义：
This is because the language spec (ECMA 334 v4, 15.8.4) (before C# 5) defines:
{
E e = ((C)(x)).GetEnumerator();
try {
V v;
while (e.MoveNext()) {
v = (V)(T)e.Current;
embedded-statement
}
}
finally {
… // Dispose e
}
}
注意变量 v 您的列表）被声明为 。因此，通过捕获变量的规则，列表的所有迭代将共享捕获的变量持有人。
Note that the variable v (which is your list) is declared outside of the loop. So by the rules of captured variables, all iterations of the list will share the captured variable holder.
从C＃5开始，改变：迭代变量c $ c> v ）作用域在内。我没有规范参考，但它基本上变成：
From C# 5 onwards, this is changed: the iteration variable (v) is scoped inside the loop. I don't have a specification reference, but it basically becomes:
{
E e = ((C)(x)).GetEnumerator();
try {
while (e.MoveNext()) {
V v = (V)(T)e.Current;
embedded-statement
}
}
finally {
… // Dispose e
}
}
重新取消订阅如果你主动要取消订阅一个匿名处理程序，诀窍是捕获处理程序本身：
Re unsubscribing; if you actively want to unsubscribe an anonymous handler, the trick is to capture the handler itself:
EventHandler foo = delegate {...code...};
obj.SomeEvent += foo;
...
obj.SomeEvent -= foo;
同样，如果你想要一个一次性的事件处理程序p>
Likewise, if you want a once-only event-handler (such as Load etc):
EventHandler bar = null; // necessary for "definite assignment"
bar = delegate {
// ... code
obj.SomeEvent -= bar;
};
obj.SomeEvent += bar;
现在是自动取消订阅;-p
This is now self-unsubscribing ;-p
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