Question

I’m trying to pass two values on Button_Click Event

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();
}

I’m using visual studio 2010 c#. On the panel each Button’s text values are all different. But the the buttons’ Click_Events are acting all the same. Could someone tell me which part of code i am getting this logical error?

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Answer 1

Looks like a common pitfall with enumerators. Basically if you use the enumerator variable (expense in this case) for a lambda, it always creates a closure over the same variable, so it always uses the same value. You can fix it like this:

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);
}

You can think of your lambda as being passed a reference to the variable expense. Even though the value of the variable changes with each iteration, the reference still points to the same variable. That is why it helps creating a locally scoped variable for each iteration (currentExpense). The string value and also the location are different because they are assigned to another location (Button.Text, Button.Location) each iteration.