Markup Extensions in Silverlight have to be public

In the good-to-know category we today have a problem out of the Silverlight 5 world, where Markup Extensions are now possible. Beware though: If the class is not public Silverlight will simply freeze while starting without giving a proper error message. So always be sure to make your extensions public.

Good to know and would have saved me quite some time if I knew that beforehand.