I think that the best way to achieve the validation functionality is to create your own DropDownButton by inheriting ours and then add an IsValid property on which change you'll set the correct VisualState.
I've created a sample project with a DataForm, TreeViews and DropDownButtons so could you please examine it and see if this is what you're looking for? In the TreeView you'll see Items and Invalid Items and if you select an Invalid Item the dropdownbutton should become red and if you focus it it's error tooltip should appear.
If you have more questions please feel free to ask.
the Telerik team
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