>But this error happened even the baseclass is a form! You can try this out with Mark E. McCasland's frmcabuilder in cabuilder tools's new version.

It doesn't matter what the base class is. What matters is if you defined it in a PRG or a VCX. You cannot save a class defined in a PRG as a VCX. As far as Mark's tool, the most recent version I could find here is from November 2003, and I couldn't find anywhere in his code where he's using SaveAsClass. If you have a more recent version that demonstrates this, please send it to me.