https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.maskedtextbox.maskfull?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-6.0
MaskedTextBox.MaskFull Property (System.Windows.Forms) | Microsoft Learn
Gets a value indicating whether all required and optional inputs have been entered into the input mask.
property systemwindows formsmaskedtextboxmicrosoftlearn