Question

I have a List<string> and I want to iterate over this collection and do something with each string on a button click. I have a small example here to illustrate what I’m trying to do:

//items is a System.Collections.Generic.List<string>
foreach (string s in items)
{
    Button b = new Button() { Content = s };
    b.Click += (obj, ev) =>
    {
        MessageBox.Show(s);
    }
    //add b to form, container, etc...
}

As you would expect the buttons are created appropriately with the correct content, however when I click any of the buttons, the text inside the MessageBox is always the last string in items. What am I missing with this? Why is it that all the Click functions for the buttons are passed the last item in the collection?

Answer 1

The foreach loop is changing s, which is used in the lambda. The lambda uses the current value of s at the point of execution, not declaring it (in techspeak: “closures close over variables, not values”). You’ll have to make a local variable:

foreach (string s in items)
{
    string local = s;
    Button b = new Button() { Content = s };
    b.Click += (obj, ev) =>
    {
        MessageBox.Show(local);
    }
    //add b to form, container, etc...
}

Thus you have a reference to the instance of s at the point of decleration, not execution.

Eric Lippert has two fantastic articles about it: part 1, part 2.