https://learn.microsoft.com/es-es/dotnet/api/system.windows.forms.form.activemdichild?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-9.0
Form.ActiveMdiChild Property (System.Windows.Forms) | Microsoft Learn
Gets the currently active multiple-document interface (MDI) child window.
property systemwindows formsmicrosoftlearn