C# 事件处理程序委托中的闭包? [英] Closures in C# event handler delegates?

问题描述

目前我来自函数式编程背景，如果我不了解 C# 中的闭包，请见谅.

I am coming from a functional-programming background at the moment, so forgive me if I do not understand closures in C#.

我有以下代码来动态生成获取匿名事件处理程序的按钮:

I have the following code to dynamically generate Buttons that get anonymous event handlers:

for (int i = 0; i < 7; i++)
{
    Button newButton = new Button();

    newButton.Text = "Click me!";

    newButton.Click += delegate(Object sender, EventArgs e)
    {
        MessageBox.Show("I am button number " + i);
    };

    this.Controls.Add(newButton);
}

我希望文本 "I am button number" + i 在 for 循环的迭代中以 i 的值关闭.但是，当我实际运行程序时，每个 Button 都会显示 I am button number 7.我错过了什么?我用的是VS2005.

I expected the text "I am button number " + i to be closed with the value of i at that iteration of the for loop. However, when I actually run the program, every Button says I am button number 7. What am I missing? I am using VS2005.

所以我想我的下一个问题是，我如何捕获价值?

So I guess my next question is, how do I capture the value?

推荐答案

要获得这种行为，你需要将变量复制到本地，而不是使用迭代器:

To get this behavior, you need to copy the variable locally, not use the iterator:

for (int i = 0; i < 7; i++)
{
    var inneri = i;
    Button newButton = new Button();
    newButton.Text = "Click me!";
    newButton.Click += delegate(Object sender, EventArgs e)
    {
        MessageBox.Show("I am button number " + inneri);
    };
    this.Controls.Add(newButton);
}

在这个问题中更详细地讨论了推理.

The reasoning is discussed in much greater detail in this question.

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