The class foo can be designed, but is not the first class in the file error

Question

I have this

The class foo can be designed, but is not the first class in the file

error after adding foo class derived from TextBox

Answer 1

The error message is pretty clear. You have a .cs file containing multiple classes. Move each class to it's own file and you should be fine.

EDIT: If you insist on having all classes in the same file, then put the foo class at the top. Order matters- by convention, the designer uses the first class in the file for controls. You cannot have multiple control classes in the same file without seeing this error.