Thank you for the question.
RadComboBox and RadListBox underwent a major refactor of the data binding logic and the events for Q3 2009. We attempted to make the behavior of RadComboBox and RadListBox as close as possible to the standard ComboBox and ListBox controls. You will notice that you can not set the Text of the standard ComboBox control in the SelectedIndexChanged event as well. There is an internal synchronization between the combo box text and its items and it happens after the SelectedIndexChanged event has fired. In RadComboBox a similar thing is happening.
You will have to find a more appropriate moment to update your text. Write again if you have other questions.
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