class MyComparer : IEqualityComparer<User>
    {
        public bool Equals(User x, User y)
        {
            if (x == null && y == null)
            {
                return false;
            }
            else
            {
                return x.username == y.username;
            }
        }

        public int GetHashCode(User obj)
        {
            return obj.username.GetHashCode();
        }
    }