Thank you for providing the code.
As I test I compared the behavior of RadComboBox to that of the classic TreeView control.
If you put a classic ASP.NET TreeView which loads its Nodes via AJAX callbacks and expand a Node, the ServerValidate method still fires. This leads to the conclusion that this behavior is inherent to ASP.NET.
In this case, I suggest you put a check for !Page.IsCallback
in the ServerValidate
event handler to distinguish normal from AJAX (which are the invalid ones) cases.
Should you have any further questions, please feel free to contact us again.
All the best,
the Telerik team