In order to achieve this you will have to edit the template of the RadComboBox. The easiest way is with Expression Blend. Just drop a ComboBox into the design surface, right-click it and choose from the menu Edit Template -> Edit a Copy
. Then Blend will generate all the necessary resources. Find the PopUp element and add the header into the template of the PopUp after the Clear selection button. After that you can use the ItemTemplate
to template the same way the items. For more information you can refer also to this
Hope this information is helpful.
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