public void Update()
{
    int count = db.users.Count(u => u.userid == userid);

    if (count == 0) // no user found
    {
        throw new Exception(String.Format("User not found for id {0}.", userid));
    }
    else if (count > 1) // more than 1 user found
    {
        throw new Exception(String.Format("Multiple users found for id {0}.", userid));
    }

    var user = db.users.FirstOrDefault(u => u.userid == userid);

    user.emailaddress = emailaddress;

    if (!String.IsNullOrEmpty(password))
    {
        user.password = password;
    }
    user.storeid = storeid;
    user.role = role;

    db.SaveChanges();
}