Loading Active Directory users by username and by SID
I have limited knowledge in C#, but could you not extract something like this?

private static ADUser InternalLoad( IdentityType type, string identity ) {
    using (PrincipalContext pc = MyUtilities.GetPrincipalContext())
    using (UserPrincipalExtension user = UserPrincipalExtension.FindByIdentity(pc, type, identity))
    {
        if (user != null)
        {
            return Utilities.LoadUser(user);
        }
    }

    return null;
}