sorting an array of customer class inside another class

Question

im trying to sort the array of customers that is in the class of CustomerArray.

the customer class have a default Sort by implementing the IComparable interface, which compare by the customer id.

when trying to sort the array with the name of the customer it makes a NullException.

how to solve please

public class Customer :IComparable
{
    public int id;
    public string name;

    public override string ToString()
    {
        return string.Format("{0}) {1,7}", id, name);
    }


    public int CompareTo(object obj)
    {
        return this.id.CompareTo(((Customer)obj).id);
    }
}

public class CustomerArray : IEnumerable,IComparer<Customer>
{
    public Customer[] allCustomers;
    private int currentSize = 4;
    private int CustomerCount = 0;

    public CustomerArray() { allCustomers = new Customer[4]; }

    public void Add(Customer newCustomer)
    {
        if (CustomerCount >= allCustomers.Length)
        {
            Customer[] temp = (Customer[])allCustomers.Clone();
            currentSize *= 2;
            allCustomers = new Customer[currentSize];
            Array.Copy(temp, allCustomers, temp.Length);
            temp = null;
        }

        allCustomers[CustomerCount++] = newCustomer;
    }

    public void Sort(IComparer<Customer> comp)
    {
        Array.Sort(this.allCustomers,comp);
    }

    //public void Sort()
    //{
    //    Array.Sort(allCustomers);
    //}

    public Customer this[int index]
    {
        get 
        {
            if (index < 0 || index >= allCustomers.Length)
            {
                throw new IndexOutOfRangeException();
            }

            return allCustomers[index];
        }
        set 
        {
            if (index < 0 || index >= allCustomers.Length)
            {
                throw new IndexOutOfRangeException();
            }

            allCustomers[index] = value;
        }
    }

    public Customer this[string name]
    {
        get
        {
            foreach (Customer c in allCustomers)
            {
                if (c.name == name)
                    return c;
            }
            throw new IndexOutOfRangeException();
        }

        set
        {
            for (int i = 0; i < allCustomers.Length; i++)
            {
                if (allCustomers[i].name == name)
                {
                    allCustomers[i] = value;
                    return;
                }
            }

            throw new IndexOutOfRangeException();
        }
    }

    public IEnumerator GetEnumerator()
    {
        foreach (Customer c in allCustomers)
        {
            yield return c;
        }
    }

    public int Compare(Customer x, Customer y)
    {
        return x.name.CompareTo(y.name);
    }
}

public class CompareBy : IComparer<Customer>
{

    public int Compare(Customer x, Customer y)
    {
        return x.name.CompareTo(y.name);//when running a  NullReferenceException() have thowen why ? and how can i solve it?
    }
}


class Program
{
    static void Main(string[] args)
    {
        CustomerArray cs = new CustomerArray();
        cs.Add(new Customer { id = 1, name = "khalil" });
        cs.Add(new Customer { id = 3, name = "wael" });
        cs.Add(new Customer { id = 6, name = "dani" });
        cs.Add(new Customer { id = 2, name = "alfred" });
        cs.Add(new Customer { id = 4, name = "daad" });
        cs.Add(new Customer { id = 5, name = "fadi" });

        //

        cs.Sort(new CompareBy());//the error is here
                                 //trying to sort using name

        foreach (Customer c in cs)
            Console.WriteLine(c);

        Console.ReadLine();
    }           
}

    

    public class Customer :IComparable
    {
        public int id;
        public string name;

        public override string ToString()
        {
            return string.Format("{0}) {1,7}", id, name);
        }


        public int CompareTo(object obj)
        {
            return this.id.CompareTo(((Customer)obj).id);
        }
    }

    public class CustomerArray : IEnumerable,IComparer<Customer>
    {
        public Customer[] allCustomers;
        private int currentSize = 4;
        private int CustomerCount = 0;

        public CustomerArray() { allCustomers = new Customer[4]; }

        public void Add(Customer newCustomer)
        {
            if (CustomerCount >= allCustomers.Length)
            {
                Customer[] temp = (Customer[])allCustomers.Clone();
                currentSize *= 2;
                allCustomers = new Customer[currentSize];
                Array.Copy(temp, allCustomers, temp.Length);
                temp = null;
            }

            allCustomers[CustomerCount++] = newCustomer;
        }

        public void Sort(IComparer<Customer> comp)
        {
            Array.Sort(this.allCustomers,comp);
        }

        //public void Sort()
        //{
        //    Array.Sort(allCustomers);
        //}

        public Customer this[int index]
        {
            get 
            {
                if (index < 0 || index >= allCustomers.Length)
                {
                    throw new IndexOutOfRangeException();
                }

                return allCustomers[index];
            }
            set 
            {
                if (index < 0 || index >= allCustomers.Length)
                {
                    throw new IndexOutOfRangeException();
                }

                allCustomers[index] = value;
            }
        }

        public Customer this[string name]
        {
            get
            {
                foreach (Customer c in allCustomers)
                {
                    if (c.name == name)
                        return c;
                }
                throw new IndexOutOfRangeException();
            }

            set
            {
                for (int i = 0; i < allCustomers.Length; i++)
                {
                    if (allCustomers[i].name == name)
                    {
                        allCustomers[i] = value;
                        return;
                    }
                }

                throw new IndexOutOfRangeException();
            }
        }

        public IEnumerator GetEnumerator()
        {
            foreach (Customer c in allCustomers)
            {
                yield return c;
            }
        }

        public int Compare(Customer x, Customer y)
        {
            return x.name.CompareTo(y.name);
        }
    }

    public class CompareBy : IComparer<Customer>
    {

        public int Compare(Customer x, Customer y)
        {
            return x.name.CompareTo(y.name);//when running a  NullReferenceException() have thowen why ? and how can i solve it?
        }
    }
    
   
    class Program
    {
        static void Main(string[] args)
        {
            CustomerArray cs = new CustomerArray();
            cs.Add(new Customer { id = 1, name = "khalil" });
            cs.Add(new Customer { id = 3, name = "wael" });
            cs.Add(new Customer { id = 6, name = "dani" });
            cs.Add(new Customer { id = 2, name = "alfred" });
            cs.Add(new Customer { id = 4, name = "daad" });
            cs.Add(new Customer { id = 5, name = "fadi" });

            //

            cs.Sort(new CompareBy());//the error is here
                                     //trying to sort using name

            foreach (Customer c in cs)
                Console.WriteLine(c);
           
            Console.ReadLine();
        }           
    }
}