Thank you for contacting us.
Unfortunately, RadComboBox for Silverlight does not support such feature out of the box. However, I happened to have 2 similar projects. In one of them the style of the ComboBox is changed, so that a Header can be displayed and in the other instead of a ComboBox it is used a DropDown Button
in which a GridView is put.
Hope the projects will help. If you have any other questions regarding our controls please let us know.
the Telerik team
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