I am using UserController.GetUserByUserName(), and whether I pass True or False to isHydrated, the results remain the same. When I analyzed core SqlDataProvider, I found that after setting isHydrated to False, the Membership Property of UserInfo should not have been hydrated.

Is that true?? If yes, then why is still Membership being filled even after setting isHydrated to False?

Secondly, I want to find out, whether a User exists across any Portal. I sent the username to this function (UserController.GetUserByUserName) & set the PortalId to -1. I indeed got a UserInfo object even if that user is not registered with the current portal. Is that the intended behavious of this function, or a side-effect of some other mistake I am making in the process???

The read only property Membership of the UserInfo class implements a "lazy loading" or "load on first access" pattern such that when the Membership property is first referenced in your code, its data (as contained in the UserMembership class) WILL be automatically hydrated if it has not already been hydrated when the UserInfo class data was hydrated (such as when calling GetUserByUserName with IsHydrated set to TRUE.  Thus, it only appears that the Membership property has been hydrated when the IsHydrated parameter of GetUserByUserName was set to FALSE.

The GetUsersByUserName method seems to be returning the correct information - that is any users satisfying the Username match - regardless of PortalId when the PortalId parameter is set to -1.  Is you want to check registration within the current portal, you would have to supply the current PortalId.