WinForms ComboBox data binding gotcha *

Question

Ok, I'm a web developer, but I find myself dabbling in windows forms projects every now and then. One thing that confounds me to this day is the following.

Assume you are doing something like the following

List<string> myitems = new List<string>
{
    "Item 1",
    "Item 2",
    "Item 3"
};

ComboBox box = new ComboBox();
box.DataSource = myitems;

ComboBox box2 = new ComboBox();
box2.DataSource = myitems

So now we have 2 combo boxes bound to that array, and everything works fine. But when you change the value of one combo box, it changes BOTH combo boxes to the one you just selected.

Now, I know that Arrays are always passed by reference (learned that when i learned C :D), but why on earth would the combo boxes change together? I don't believe the combo box control is modifying the collection at all.

As a work around, don't this would achieve the funcionality that is expected / desired

ComboBox box = new ComboBox();
box.DataSource = myitems.ToArray();

Answer

This has to do with how data bindings are set up in the dotnet framework, especially the BindingContext. On a high level it means that if you haven't specified otherwise each form and all the controls of the form share the same BindingContext. When you are setting the DataSource property the ComboBox will use the BindingContext to get a ConcurrenyMangager that wraps the list. The ConcurrenyManager keeps track of such things as the current selected position in the list.

When you set the DataSource of the second ComboBox it will use the same BindingContext (the forms) which will yield a reference to the same ConcurrencyManager as above used to set up the data bindings.

To get a more detailed explanation see BindingContext.

< br > via < a class="StackLink" href=" http://stackoverflow.com/questions/482/" >WinForms ComboBox data binding gotcha< /a>

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