Your approach would not work because Items that are loaded in the ComboBox are not sent back to the server and subsequently to the ItemsRequested event handler.
So, upon each callback the 'Unknown' Item will not be found in the collection and will be added again.
In order to work around this behavior please do the following:
- Add the Item on the first page load:
|if ( ! this.IsPostBack)
| this.rcbBeginPeriod.Items.Add(new RadComboBoxItem("<Unknown>", "-1"));
- Clear the existing Items in the ComboBox in the beginning of the ItemsRequested event handler:
|protected void rcbBeginPeriod_ItemsRequested(object o, RadComboBoxItemsRequestedEventArgs e)
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