You have to wrap it in parentheses:
roles.Contains(u.RoleID.Value) && (u.isValid ?? false)
bit of confused with (u.isValid ?? false), does this not mean that if
u.isValid == null then make it false and look for users where
u.isValid is false, this is not what I want.
No, it just means that nulls are treated as false and that all users are taken which isValid is neither null nor false. It works because the ??-operator converts the Nullable to a bool, so you can use it with &&. I don't like it, i prefer explicit code that i understand later:
roles.Contains(u.RoleID.Value) && u.isValid.HasValue && u.isValid.Value
or simpler by using the ==-operator with the bool?:
roles.Contains(u.RoleID.Value) && u.isValid == true
