Increment letters

IncrementLetters.cs

using System;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                Console.Write("Enter letter series: ");
                var input = Console.ReadLine();
                Console.WriteLine(IncrementLetter(input));
            }
            
            
        }
        
        /**
        * Will increment capital letter input, 
        * eg A -> B, AA -> AB, ZZ -> AAA
        **/

        private static string IncrementLetter(string input)
        {
            char[] inputArray = input.ToCharArray();

            for (int i = inputArray.Length - 1; i >= 0; i--)
            {
                if (inputArray[i] == 'Z')
                {
                    if (i == 0)
                    {
                        return new string('A', inputArray.Length + 1); // Will print A the length of input string + 1
                    }
                    else
                    {
                        inputArray[i] = 'A';
                    }
                }
                else
                {
                    inputArray[i] = (char) (inputArray[i] + 1);

                    return new string(inputArray);
                }
            }

            return "";

        }
    }
}