How to set d:DesignInstance to a generic type?
Generics are not supported yet. We can derive a class from any generics and use that class instead.
Oops, please see comment below from "Jan 'splite'K". It sounds that there is a markup extension that could do the job.
Also, look at uli78 link
You could try this:
<DataGrid d:DataContext="{d:DesignInstance Type={x:Type om:ObservableCollection`1}}" />