字符串分解为字符，字符分别转为ascii对应数字，最后数字连接起来转为字符串，具体如下：

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

namespace ConsoleApp1
{
   public  class b
    {
        public static string StringTInt(string input)
        {
            char[] values = input.ToCharArray();
            string newtext = "";
            foreach (char letter in values)
            {
                // Get the integral value of the character.
                int value = Convert.ToInt32(letter);
                // Convert the decimal value to a hexadecimal value in string form.
                string hexOutput = String.Format("{0:X}", value);
                Console.WriteLine("Hexadecimal value of {0} is {1}", letter, hexOutput);

                newtext= newtext + Convert.ToString(value);   
            }
            Console.WriteLine(newtext);
            //Console.ReadLine(); 
          return newtext; 
        }
    }
    class c 
    { 
        static void Main()
        { 
            string a = "abc";
            string c = b.StringTInt(a);
            Console.WriteLine($"\"{a}\"转为数字为：{c}");
            Console.ReadLine();
        }
    }

}