How to Enable and Disable the menustrip in mdiparent

Make your menustrip modifier to "Public" and use below code in child form to disable menustrip.

Form1 parent = (Form1)this.MdiParent;
//parent.menuStrip1.Visible = false;
parent.menuStrip1.Enabled= false;

Thanks

In Form_load event of the child, disable menu or better yet, fire custom event ChildOpened. In FormClosed event, fire another event ChildClosed that will be caught in the parent form.

You can avoid first one by disabling the menu when you instantiate child form, but before opening it.

Based on MdiChildren collection you can then enable/disable menu with

menu.Enabled = (this.MdiChildren.Count == 0);