The SelectedIndexChanged event is raised every time the SelectedIndex property value changes. If your ComboBox is initially empty then its SelectedIndex is -1. When you bind data to it the first item is selected by default and the SelectedIndex changes to 0. You are then explicitly setting the SelectedIndex to -1 again.

If you're only interested in knowing when the user makes a selection then handle the SelectionChangeCommitted event instead.

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