See for example : How to Enable and Disable the menustrip in mdiparent
How to disable/enable Menu Strip?

I have a mdi parent form, let's call it MainForm, which has menustrip. Each menu button opens a child form.
I have a child form called LoginForm
When MainForm is loaded, LoginForm will show up. But at the meanwhile, I would like to disable menustrip or gray out menu strip, whatever it is called, user shouldn't do anything before logging in.
So how should I write the code(in MainForm) to disable menu strip?
After successfully logging in, the menu strip should be enabled. So this piece of code will be in LoginForm. How should I write the code (in Login Form) to enable menu strip? The issue is: menu strip belongs to MainForm, how can LoginForm code reaches MainForm menu strip?
Thanks.