The designer could not be shown for this file because none of the classes within it can be designed

You can also try doing this:

• Close all the UI design pages
• Clean Solution
• Build Solution
• Open the desired UI design pages

This might or might not help but it certainly resolved the same issue in my project.

Here's another possible solution:

Under the project properties under build, my platform target was set to x64. I updated this to "Any CPU", rebuilt my project and the designers opened fine.

This explains it better: Visual studio designer in x64 doesn't work

Your best bet would be to start with a version of OurServiceBase with no functionality and see if you can design MyService. If so, then slowly add back functionality until it breaks.

Since it looks like Visual Studio is having a problem serializing one of the members of OurServiceBase.