FxCop wants me to spell Username with a capital N (i.e. UserName), due to it being a compound word. However, due to consistency reasons we need to spell it with a lowercase n -
In the custom dictionary that comes with FxCop (located in my system in C:\Program Files\Microsoft FxCop 1.36\CustomDixtionary.xml, but YMMV) in Words\Compounds has a username entry. Delete it. You still need the discrete exception.