"XXX. Index "does not extend class" System.Web.UI.Page ", so the issue is not allowed here

"XXX. Index "does not extend class" System.Web.UI.Page ", so the issue is not allowed here

Cause: The path of the design page inherits and the background. CS Page class path inconsistency caused by

See

This is the style of the design page

This is the background CS file code

If the paths in these two locations are inconsistent, the above errors will occur

Workaround: Unify the paths of these two locations to resolve

"XXX. Index "does not extend class" System.Web.UI.Page ", so the issue is not allowed here