在 System.Threading.ThreadHelper.ThreadStart()</StackTrace><ExceptionString>System.InvalidOperationException: The model backing the 'DBtest' context has changed since the database was created. Either manually delete/update the database, or call Database.SetInitializer with an IDatabaseInitializer instance. For example, the DropCreateDatabaseIfModelChanges strategy will automatically delete and recreate the database, and optionally seed it with new data.