Question from one example in Item 5 《Effective C#》

1. Customer.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CSharp
{
    class Customer:IFormattable
    {
        public string Name { get; set; }
        public decimal Revenue { get; set; }
        public string ContactPhone { get; set; }

        string IFormattable.ToString(string format, IFormatProvider formatProvider)
        {
            if (formatProvider != null)
            {
                ICustomFormatter fmt = formatProvider.GetFormat(this.GetType()) as ICustomFormatter;
                if (fmt != null)
                {
                    return fmt.Format(format, this, formatProvider);
                }
            }

            switch (format)
            {
                case "r":
                    return Revenue.ToString();
                case "p":
                    return ContactPhone;
                case "np":
                    return string.Format("{0,20}, {1,10:c}", Name, Revenue);
                case "n":
                case "G":
                default:
                    return Name;
            }
        }
    }

    public class CustomFormatter : IFormatProvider
    {
        /// <summary>
        ///
        /// </summary>
        /// <param name="formatType"></param>
        /// <returns></returns>
        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter))
                return new CustomerFormatProvider();
            return null;
        }

        //Nested class
        private class CustomerFormatProvider : ICustomFormatter
        {
            public string Format(string format, object arg, IFormatProvider formatProvider)
            {
                Customer c = arg as Customer;
                if (c == null)
                {
                    return arg.ToString();
                }
                return string.Format("{0,50}, {1,15}, {2,10:C}", c.Name, c.ContactPhone, c.Revenue);
            }
        }
    }
}

2. Program.cs

   class Program
    {
        static void Main(string[] args)
        {

            Customer c1 = new Customer();
            c1.Name = "qiqi";
            c1.Revenue = 13000;
            c1.ContactPhone = "1111111";
            Console.WriteLine(string.Format(new CustomFormatter(), "", c1)); 

            // Expected: call the nested class CustomerFormatProvider::Format in CustomFormatter.

            //    return                                 qiqi,             1111111, ￥13,000.00

            //   But actual result is: empty.

            //   Why?

        }

     }