Hello Richard Koslik,
The RadGridView's validation logic will not get executed when the item is edited outside the gridview.
Basically, you need an event which fires before the selection is made in RadGridView. In Q3 2010 which will be released later this week, we are introducing a new event - SelectionChanging
which can be canceled if you do not want to change the current selected item.
Hope this helps.
the Telerik team
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