Cannot show one or more base types because the following types cannot be found: '<element>'
This error is raised when Class Designer cannot find the location of a source type or when a source type no longer exists.
To correct this error
Ensure that the base type is an existing type.
If the base type is defined outside your project, ensure that you have included the correct references in Solution Explorer.
If you modify your project structure or move source files in your project, Class Designer can lose track of the type. (This is especially true of the source type of a typedef, base classes, or association types.) If this is the case, drag the modified or relocated source code to the class diagram again to redisplay it.
See Also
Concepts
Working with Visual C++ Code in Class Designer