I am getting the exact same error and the workaround discovered in this thread is the only thing that seems to work. Rebuilding the solution does not fix it.

To reiterate the error. After upgrading to mm.net 1.2 I can't open any of the .cs files in the Order System Business Object CSharp project. I get the following error, for instance when I try to open Employee.cs:

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:
Employee --- The base class 'Acme.OrderSystem.Business.ABusinessObject' cannot be designed.

But when I use right-click to edit it, I can open it. Then I change the line:

using OakLeaf.MM.Main.Business;
to:
using OakLeaf.MM.Main.Business.mmBusinessObject;

Then save it. I can then open it normally by double-clicking on it. Even after I take the mm.BusinessObject leaf back out, it opens fine. Very strange.

--- Hold on, I found it! I just took another look and I see what appears to be going on. When you right-click on the .cs file in the Solution Explorer, and select "Open with", it shows that it wants to use the CSharp "Form" editor (default). When you open it with the 2nd option (CSharp Editor), it opens. You have to make a change in order for it to "remember" what editor to use to open the file with. (You'll notice that the icon next to the .cs file changes to a different icon). So, it seems that somehow the business objects started defaulting to the "Form" editor after upgrading to mm 1.2. I'm not sure, but that's about the only thing I can think of that has changed. Other than, I'm now working on the asp.net example.

Regards, Mark

>After giving it some thought, I'm wondering if Rebuilding (vs. Building) the project would also have solved the problem. When you Build a project, it only builds what's been changed. When you Rebuild, it compiles everything.
>