In a Windows.Forms application, it’s fairly easy to make use of a MdiContainer in a Parent form and create new forms inside it’s container. However, the same result can be obtained with no MdiContainer at all.
You just create a new Form, set it’s TopLevel property to false and add it to the Controls collection of another Form.
ChildForm form = new ChildForm();
form.TopLevel = false;
ParentForm.Controls.Add( form );