https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.form.ismdichild?view=netframework-4.8.1&redirectedfrom=MSDN
Form.IsMdiChild Property (System.Windows.Forms) | Microsoft Learn
Gets a value indicating whether the form is a multiple-document interface (MDI) child form.
property systemwindows formsmicrosoftlearn