I've found an issue with the winforms designer in VS2010, exactly the same scenario works without any problems in VS2008. I've attached a repro solution for both VS2008 (WinApp08) and VS2010 (WinApp).

To repro the issues (in WinApp):

1) Try opening InherritedForm in the designer. You should get an error saying that ClassLib cannot be loaded.

There are two factors that needs to be in place for this to trigger. A component with an constructor that has a parameter using a type from another project needs to be instanciated in the constructor of a forms base form. The constructor on the baseform needs to call the constructor with this parameter (calling a different overload that does not have such a parameter does not trigger the issue). I have not tested if this only applies to components or if it also applies to controls aswell. It happens regardless of how many overloads there exists for the constructor on the component. Also if there is only one constructor, and this constructor contains such a parameter.

2) Open RegularForm. Try to drag MyExtender onto the form. You should get an error saying that ClassLib cannot be loaded.

For this to trigger it is enough that you have a constructor on the component that contains a parameter using a type from another project. I have not tested if the same applies to controls aswell.

Please note that the attached projects are repro projects. In the real project where I experience this, my extender only has a single constructor that has one such parameter.

The repro projects can be found here:

http://cid-4a7c8cdf61fa0160.skydrive.live.com/self.aspx/.Public/WinApp.zip

http://cid-4a7c8cdf61fa0160.skydrive.live.com/self.aspx/.Public/WinApp08.zip

Am I doing something I shouldn't do, or is this a bug introduced in VS2010?

Issue is also registered at connect:

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=476351