https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.tabcontrol.deselecting?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-10.0
TabControl.Deselecting Event (System.Windows.Forms) | Microsoft Learn
Occurs before a tab is deselected, enabling a handler to cancel the tab change.
event systemwindows formstabcontrolmicrosoftlearn
https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.tabcontrol.selectedindex?view=windowsdesktop-10.0
TabControl.SelectedIndex Property (System.Windows.Forms) | Microsoft Learn
Gets or sets the index of the currently selected tab page.
property systemwindows formstabcontrolmicrosoftlearn