If you are using server-side loading of items, you could an 'if' check to your code which will make sure that the number of items being loaded is not zero; if it is zero, however, you should clear the text of the RadComboBox like this:
If you, on the other hand, are loading items from a Web server you could take advantage of the OnClientItemsRequested
event where you could check the number of items being loaded from the service. Just like in the previous case, If this number is zero then you set the text of RadComboBox to empty string.
I hope this will help.
the Telerik team
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