Thank you for contacting Telerik support.
We recommend you to use RadTreeView
control for this purpose, instead of using RadComboBox
in an item template.
in the RadDropDownTree
control in code behind can be achieved by RadDropDownTree1.EmbeddedTree
. You can find more detailed information about it in our documentation article about Accessing the Embedded Tree in the DropDown
Expanding and collapsing nodes can be achieved by the methods expand()
. We would like to refer you to our article about collapsing and expanding nodes in RadTreeView
Do not hesitate to contact us if you have other questions.