Code review on Dec. 13, 2017, the original practice was on Oct. 13, 2013.

ConvertToString_CodeReview2017.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConvertToString
{
    /// <summary>
    /// code review:   
    /// API used:
    /// new string(char[])  - construct a string using the input of char array
    /// string.ToCharArray(index, length)  - convert a string to char array
    /// new char[] {'0'} - convert one char to char array
    /// (char) (n + '0') - convert one digit to char
    /// Do not use:
    /// char[] numbers = new char[]{'1','2'}; 
    /// string numberString = numbers.ToString(); 
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            RunTestcase3(); 
        }

        public static void RunTestcase1()
        {
            var result = convertTostring(123);
            Debug.Assert(result[0] == '1' && result[1] == '2' && result[2] == '3'); 
        }

        public static void RunTestcase2()
        {
            var result = convertTostring(-123);
            Debug.Assert(result[0] == '-' && result[1] == '1' && result[2] == '2' && result[3] == '3'); 
        }

        /// <summary>
        /// Learn API ToString()
        /// Dec. 13, 2017
        /// Read the question on stackoverflow.com
        /// https://stackoverflow.com/questions/27125430/array-tostring-returning-system-char-c-sharp
        /// 
        /// </summary>
        public static void RunTestcase3()
        {
            char[] numbers = new char[]{'1','2'}; 
            string numberString = numbers.ToString();

            // Figure out why the output is System.Char[]

            Debug.Assert(numberString[0] == '1'); 
        }

        /// <summary>
        /// Dec. 13, 2017
        /// </summary>
        /// <param name="n"></param>
        /// <returns></returns>
        public static char[] convertTostring(int n)
        {
            if (n < 0)
            {
                var s = convertTostring(-1 * n);
                return ("-" + new string(s)).ToCharArray(0, s.Length + 1); 
            }

            // base case
            if (n < 10)
            {
                var number = (char)(n + '0');
                return new char[]{number}; 
            }

            var rightMost = n % 10;
            var current = (char)(rightMost + '0');
 
            var next = n / 10;
            var converted = new string(convertTostring(next)) + current.ToString(); 
            return converted.ToCharArray(0, converted.Length); 
        }
    }
}