Thanks for your suggestion. I am getting closer. Still have a small issue to figure out.
When using OnClientTransferring, when I check the count of RadListBox2 it gives the count of items before the transfer.
I am allowing SelectionMode="multiple". So, if there are already 7 items in RadListBox2 and they select 5 items from RadListBox1, the code allows the transfer as the count is < 10. But, we end up with 12 items in RadListBox2.
I have tried to use get_selected to find the number of items being transfered. But, it seems that once the "transfering" starts, they are no longer selected. So, I have been unable to make that work.
I am planning on adding some code for the OnClientTransfered as that gives me an accurate count of the new number of items after the transfer. But, at this point it is not possible to use the set_cancel(true). So, I will just have to write code that deletes the extra times.
If anyone has a better way to detect the number of items being transfered + the number of items currently in RadListBox2 to prevent more than 10 items total in RadListBox2, so that I can use the set_cancel(true), that would be a much better solution and I appreciate any assistance.