Custom Membership Provider with no username.

I have been tasked with updating our legacy membership database to use the custom membership provider.  My dilemma is that I notice many methods use the username field.  Our database uses the email address as the username field (ie there is NO username field in the DB).  Is it safe to override the default methods that use username to use email instead?

 For example:

ValidateUser(string username, string password)  would be changed to ValidateUser(string email, string password)

In addition, how would I handle methods that contain both username and email as parameters, such as CreateUser(string username, string password, string email, ...)

It seems unnecessarily redundant to input the email address for both parameters, but I see no other way.

 I guess my question is, even though I have no need for the username property, must I use it anyway?
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