Question

我正在尝试在Button_Click事件

上传递两个值

public MyClass()
{
    Int64 po = 123456;
    foreach (Expense expense in pr.Expenses)
    {
        Button btnExpenseDetail = new Button();
        btnExpenseDetail.Text = expense.ExpenseName;
        btnExpenseDetail.Location = new Point(startLocation.X + 410, startLocation.Y + (23 * 
        btnExpenseDetail.Click += (sender, e) => { MyHandler(sender, e, po , expense.ExpenseName); };            
        pnlProjectSummary_Expenses.Controls.Add(btnExpenseDetail);
    }
}  


void MyHandler(object sender, EventArgs e, string po, string category)
{
    FormExpenseDetails ed = new FormExpenseDetails(po, category);
    ed.Show();
}

我正在使用visual studio 2010 c＃。在面板上，每个Button的文本值都是不同的。但按钮'Click_Events的行为完全相同。有人能告诉我哪部分代码我得到了这个逻辑错误吗？

=============================================== =========================

Answer 1

看起来像是普查员的常见陷阱。基本上，如果对lambda使用枚举器变量（在这种情况下为expense），它总是在同一个变量上创建一个闭包，因此它总是使用相同的值。你可以像这样解决它：

foreach (Expense expense in pr.Expenses)
{
    var currentExpense = expense; // <-- This should help. Also use this variable for the lambda.
    Button btnExpenseDetail = new Button();
    btnExpenseDetail.Text = currentExpense .ExpenseName;
    btnExpenseDetail.Location = new Point(startLocation.X + 410, startLocation.Y + (23 * 
    btnExpenseDetail.Click += (sender, e) => { MyHandler(sender, e, po , currentExpense.ExpenseName); };            
    pnlProjectSummary_Expenses.Controls.Add(btnExpenseDetail);
}

您可以将lambda视为传递对变量expense的引用。即使变量的值随每次迭代而变化，引用仍指向同一变量。这就是为什么它有助于为每次迭代（currentExpense）创建一个本地范围的变量。字符串值以及位置不同，因为每次迭代都会将它们分配给另一个位置（Button.Text，Button.Location。

Answer 2

此代码应该有效：

public MyClass()
{
    Int64 po = 123456;
    foreach (Expense expense in pr.Expenses)
    {
        var expenseName = expense.ExpenseName;
        Button btnExpenseDetail = new Button();
        btnExpenseDetail.Text = expense.ExpenseName;
        btnExpenseDetail.Location = new Point(startLocation.X + 410, startLocation.Y + (23 * 
        btnExpenseDetail.Click += (sender, e) => { MyHandler(sender, e, po, expenseName); };            
        pnlProjectSummary_Expenses.Controls.Add(btnExpenseDetail);
    }
}  


void MyHandler(object sender, EventArgs e, string po, string category)
{
    FormExpenseDetails ed = new FormExpenseDetails(po, category);
    ed.Show();
}

让我们看一些更基本的东西。

static void Main(string[] args)
{
    var qs = new List<Action>();

    for (var i = 0; i < 10; i++)
        qs.Add(() => f("doer", i));

    for (var i = 0; i < 10; i++)
        qs[i]();

}

private static void f(string x, int y)
{
    Console.WriteLine("{0}: {1}", x, y);
}

当你运行上面的代码时，你总是得到输出：“doer：10”。让我们反编译那段代码：

private static void f(string x, int y)
{
    Console.WriteLine("{0}: {1}", x, y);
}

private static void Main(string[] args)
{
    List<Action> qs = new List<Action>();
    <>c__DisplayClass1 CS$<>8__locals2 = new <>c__DisplayClass1();
    CS$<>8__locals2.i = 0;
    while (CS$<>8__locals2.i < 10)
    {
        qs.Add(new Action(CS$<>8__locals2.<Main>b__0));
        CS$<>8__locals2.i++;
    }
    for (int i = 0; i < 10; i++)
    {
        qs[i]();
    }
}

[CompilerGenerated]
private sealed class <>c__DisplayClass1
{
    // Fields
    public int i;

    // Methods
    public void <Main>b__0()
    {
        Program.f("doer", this.i);
    }
}

如您所见，编译器生成了一个名为c__DisplayClass1的类，并在进入循环之前对其进行了初始化。之后，它只增加了变量i的{{1}}属性。

因此，当我在下一个循环中调用theese lambdas时，它使用CS$<>8__locals2对象来查看内部变量。

（我的英语不是很好解释它，但它就在那里......）

Answer 3

这与C＃＆lt; = 4如何处理foreach循环有关。基本上，实例开销是在循环外定义的，然后有一个内部循环，它将指针更改为下一个项目。像这样的伪代码：

Expense expense;
for expense in pr.Expenses
   // do processing

如果您根据引用来考虑，引用的值，费用指向，迭代期间的更改。因此，当您的点击事件触发时，它指向最后一个项目。现在，这应该在c＃5中得到修复，并且已经就此进行了讨论。

修复很简单：

Int64 po = 123456;
foreach (Expense expense in pr.Expenses)
{
    var localExpense = expense;
    Button btnExpenseDetail = new Button();
    btnExpenseDetail.Text = expense.ExpenseName;
    btnExpenseDetail.Location = new Point(startLocation.X + 410, startLocation.Y + (23 * 
    btnExpenseDetail.Click += (sender, e) => { MyHandler(sender, e, po , localExpense.ExpenseName); };            
    pnlProjectSummary_Expenses.Controls.Add(btnExpenseDetail);
}

将值传递给动态按钮的事件处理程序

3 个答案: