关闭C＃事件处理程序代理？ [英] Closures in C# event handler delegates?

问题描述

我现在来自功能编程背景，所以原谅我，如果我不明白C＃中的闭包。

我有以下代码动态生成获取匿名事件处理程序的按钮：

  for（int i = 0; i< 7; i ++）
 {
 Button newButton = new Button（）; 
 
 newButton.Text =点击我！; 
 
 newButton.Click + = delegate（Object sender，EventArgs e）
 {
 MessageBox.Show（I'm button number+ i）; 
}; 
 
 this.Controls.Add（newButton）; 
} 
  

我预计文字我是按钮编号在for循环的迭代中，+ i 被关闭，值为 i 。但是，当我实际运行程序时，每个按钮都说我是按钮号码7 。我失踪了什么我正在使用VS2005。

编辑：所以我想我的下一个问题是，如何捕获这个值？

解决方案

要获得此行为，您需要在本地复制变量，而不是使用迭代器：

  for（int i = 0; i< 7; i ++）
 {
 var inneri = i; 
按钮newButton = new Button（）; 
 newButton.Text =点击我！; 
 newButton.Click + = delegate（Object sender，EventArgs e）
 {
 MessageBox.Show（我是按钮号+ inneri）; 
}; 
 this.Controls.Add（newButton）; 
} 
  

更详细地讨论推理，此问题。

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

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);
}

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.

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

解决方案

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);
}

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

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