how to inherit the base form in derived form

Ah, this just seems to be a VS bug (or feature ?). You should have a blue squiggly line underneath the inheritance.frmbase line. This should give you the option of changing the declaration to exactly the same thing inheritance.frmbase. Just select that and the error should resolve itself.

Hope this helps

With VB in WinForms, you actually have two partial classes: one with your code, and one with the automatically generated designer code. When you go to the code of a form, you see where your code is written. From that page, go to the drop-down in the upper right and note the two methods that are grayed out, Dispose(boolean) and InitializeComponent. Select either of those, and you are taken to the other code module, with the code that VS generates automatically to manage the form.

Both of these modules must inherit from the same class. When they do not, the compiler cannot merge the modules into the single form class, and you get your error. So after setting the Inherits in your code module, go to the designer module and change Inherits System.Windows.Forms.Form to Inherits frmmain. Harmony will be restored.