Why I got extra close button on mdi child window?

Question

I got a strange problem. My mdi child form has 2 close buttons and 2 maximized buttons.

A screenshot of the problem:

Answer 1

It's a bug in Winforms. This will happen when the child is created by the parent's constructor. Move it to the Load event.

Answer 2

try this:

childform.ControlBox = false;