I confess that I am very stubborn about a few issues when it comes to Windows Phone development; support themes, support landscape, provide design time support. Recently I’ve been using MVVM Light to develop an application and it has great support for design time data. However, at some random point during the development the designer failed to show the sample data. This isn’t the first time I’ve seen this so I decided to have a dig and find out what was going on. Basically it’s flaky, very flaky. At first the problem seemed to be with one type of sample data I was creating. When I commented the code out, the sample data appeared, put it back the sample data vanished. So I thought I was onto something. Just a red-herring. Eventually, after I left it for a period of time, the sample data vanished and stayed away regardless of what I removed. I then looked at Task Manager for clues. Sure enough there were two instances of ‘Microsoft Visual Studio XAML UI Designer’ running, one for each xaml project I had opened. So I killed both of them off, and returned to my problem xaml page. Selected ‘reload designer’ and the sample data sprang back to life. As I said, flaky. So in summary, I still don’t know what the problem is, but killing the UI designer and reloading a new one seems to resolve it, plus you don’t have to restart VS to do that. If you do know the answer to this then please let me know!
Aha! Turns out the bulk of my problems was an issue with the sample data. Essentially when I created my sample data I had inadvertently caused a null exception. However, rather then show any error the designer had gracefully eaten the exception and sulked. So, if your data vanishes, check out your sample data for errors.