Use the Close All MDI Forms command from the Window menu to close all open Multiple Document Interface (MDI) forms.
To close all open MDI forms
Select the Close All MDI Forms command from the Window menu.
A confirmation dialog is then displayed, prompting you to click Yes to proceed with the closing of all MDI forms or No to abandon the request.
When you have confirmed that you do want to close all open MDI forms, they are then closed as if you have selected the Close button on each MDI child form.
If the MDI frame has no visible and enabled MDI child windows displayed, the Arrange Icons, Cascade, Close All MDI Forms, Tile Horizontal, and Tile Vertical commands are disabled.