IconColumnWidth is defined in MenuBase class which is a base class for RadMenu and RadContextMenu. So both classes have IconColumnWidth property.
The problem is that you are trying to set it to the attached property (e.g. <telerik:RadContextMenu.ContextMenu>)
Instead you should set it to the instance of RadContextMenu (e.g. <telerik:RadContextMenu IconColumnWidth="0">) the child of <telerik:RadContextMenu.ContextMenu > element
Let us know if you need more information.
the Telerik team
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