UPDATE: This issue is fixed in Service Pack 1 RTM as confirmed by a comment to this post from a MS Employee and myself testing it.
So I ran into this nice bug with Visual Studio Service Pack 1 Beta that prevented me from working with XAML designer. The exact error message is (also a screen shot at the bottom of the post):
Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)
So, I ran around and all I could find was other people having the same problem with no solution. I stumbled upon the solution though :) It's a simple workaround also...
- Close all open files in your solution that contain any xml. This includes App.Config, .xaml files, .xml files, .xsd files, etc.
- Close and reload Vistual Studio and immediately load a .xaml file and the designer will come up normally.
- Make sure you close all files that contain xml content that aren't xaml designer files when you close the solution.