To make a form owned by another form, assign its Form.Owner property a reference to the form that will be the owner.

When a form is owned by another form, it is closed or hidden with the owner form. For example, consider a form named Form2 that is owned by a form named Form1. If Form1 is closed or minimized, Form2 is also closed or hidden. Owned forms are also never displayed behind their owner form. You can use owned forms for windows such as find and replace windows, which should not disappear when the owner form is selected. To determine the forms that are owned by a parent form, use the Form.OwnedForms property.