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

We have the following shared component:

public class OurServiceBase : System.ServiceProcess.ServiceBase

This class has functionality we want in all our downstream services, such as standardized execution scheduling and logging functionality.

In a new project, I add the following:

public class MyService : System.ServiceProcess.ServiceBase

In the Windows Designer, the class shows properly.

When I change the service to derive from OurServiceBase

public class MyService : OurSharedLibrary.OurServiceBase

The designer stops working:

The full error is:
The designer could not be shown for this file because none of the classes within it can be designed. The designer inspected the following classes in the file: EmailProcessor — The base class 'OurSharedLibrary.CienaServiceBase' could not be loaded. Ensure the assembly has been referenced and that all projects have been built.

The proper assemblies are referenced, the project builds. I don't understand why the designer is flipping out over this since my service ultimately does derive from a designable class.

Any suggestions would be most welcome.

Bit more information – the call stack from the designer when it renders the error about not being able to design the derived service:

at System.ComponentModel.Design.Serialization.CodeDomDesignerLoader.EnsureDocument(IDesignerSerializationManager manager)
at System.ComponentModel.Design.Serialization.CodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager manager)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager serializationManager)
at System.ComponentModel.Design.Serialization.BasicDesignerLoader.BeginLoad(IDesignerLoaderHost host) 

7/19/2011 2:34PM EDT New discovery.

Class "OurServiceBase" exists in a separate project (usually referenced as a DLL only). On a whim, I copied the base class file into my project, built, and opened the designer. It worked! When I removed the base class file again and returned to the external DLL reference, the designer broke again.