How do I get the identifier from the user object?

How do you retrieve the identifier from the user object? Normally, I would expect the identifier to be something like this:

https:\\me.yahoo.com\bob#bob123

Looking at the user object, however, it doesn't seem like there's an equivalent. I have to use the providerUID to store my users instead.开发者_开发百科 Yesterday Google provided me with two different providerUIDs for the same user.