The ComboBox.Leave event is probably not the place you want to be doing this. This event occurs every time your mouse exits the region filled by the ComboBox. Instead, just handle the ComboBox.SelectedIndexChanged. As far as the problem, if the above suggestion doesn't solve it (indirectly), make sure the ComboBox is throwing the event correctly and step through your code - as I said before - to find where the error is. Also, make sure that your ClearSelected method isn't throwing any exceptions that would cause your code to exit prematurely before the listView1.Items[index].Selected = true is executed.
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