I was working on a list box project and wanted to use Blend to edit my container styles when I was faced with the age old problem of providing Blend with some useful data. So I created a nice little collection class wrapping my target object and asked Blend to use it as a datasource, ensuring that the collection would initialise itself in its constructor. Hurray I had a nice design view with sample data. However, when I switched back to Visual Studio it complained that the resource xaml containing my datasource definition was invalid, the dreaded AEG error. After a great deal of head scratching I realised that somehow Blend had successfully added an x:Name attribute to the resource but my collection class does not derive from any Element and therefore does not have an x:Name. Once I realised that I (ahem) hacked a quick derive-from-user control onto the collection class and we’re up and running again. So watch out, just cause Blend doesn’t complain doesn’t mean it hasn’t generated some dodgy xaml.