Convert to string - iterative solution - with bugs using those API, I should write one without using so many APIs later.

ConvertToString_Iteative_WithBugs.cs

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

namespace ConvertToStringIterative
{
    /// <summary>
    /// code review on Dec. 13, 2017
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            RunTestcase1();
            RunTestcase2(); 
        }

        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>
        /// code review on Dec. 13, 2017
        /// API used:
        /// string.Join(string delimiter, char[], start, index) - failed, it is string[] as second argument
        /// new string(char[], start, length) - ok 
        /// string.ToCharArray(start, length)
        /// 
        /// IEnumerable<char> IEnumerable.Reverse<char>
        /// Inverts the order of the elements in a sequence
        /// 
        /// Char[] IEnumerable<char>.ToArray<char>()
        /// Creates an array from a System.Collections.Generic.IEnumerable<T>
        /// </summary>
        /// <param name="n"></param>
        /// <returns></returns>
        public static char[] convertToString(int n)
        {            
            var output = new char[100]; // maximum length of integer is less than 100            

            // more edge case
            bool isNegative = n < 0;                     

            int index = 0;
            int current = isNegative? -n : n;
            
            while(true)
            {
                // base case
                if (current < 10)
                {
                    output[index] = (char)(current + '0');
                    index++; 
                    break; 
                }

                int rightMost = current % 10;

                output[index] = (char) (rightMost + '0');

                current = current / 10;                 

                index++; 
            }
            
            // reverse the order
            int length = isNegative ? (index + 1) : index;
            var converted = new string(output, 0, index);
            var reversed = converted.Reverse().ToString();
            reversed = isNegative ? ("-" + reversed) : reversed; 

            return reversed.ToArray();
        }
    }
}