If I have understood you correctly, you want to take the value of the RadComboBoxEditor when it is changed. To do so, you should subscribe to the ValueChanged event of the RadComboBoxEditor instance. This instance can be taken in the CellBeginEdit event handler by the ActiveEditor property.
I have attached a sample project to demonstrate this approach. If you have additional questions, feel free to contact me.
Best wishes,
Nikolay
the Telerik team
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