The drop down is not closed because the operation is canceled in the DropDownClosing event. It is up to you to manage the possible scenarios in this case and make sure that the user cannot perform an invalid operation.
Another approach would be to always close the drop down when the form is closed:
.radMultiColumnComboBox1.DropDownClosing -= radMultiColumnComboBox1_DropDownClosing;
I hope this will be useful. Let me know if you have additional questions.
Telerik by Progress
Check out the Windows Forms project converter, which aids the conversion process from standard Windows Forms applications written in C# or VB to Telerik UI for WinForms.For more information check out this blog post
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