The base class '<class name>' cannot be designed
This error is often preceded by the error The designer inspected the following classes in the file:.
Visual Studio loaded the class, but the class cannot be designed because the implementer of the class did not provide a designer.
To correct this error
If the class supports a designer, make sure there are no problems that would cause issues with displaying it in a designer, such as compiler errors (in the Task List). Also, make sure that all references are properly made to the class, and all class names are correctly spelled.
Otherwise, if the class is not designable, edit it in Code view.