You can use this property to switch between complete menu sets at run time. For example, you can define one System.Windows.Forms.MainMenu to be displayed when your multiple-document interface (MDI) form has no active MDI child forms and another System.Windows.Forms.MainMenu to display when a child window is displayed. You can also use a different System.Windows.Forms.MainMenu when specific conditions exist in your application that require displaying a different menu set.