using System;

using System.Linq;

using System.Text;

public class Program

{

    static void Main(string[] args)

        {

            var input = "{\"SaleToPOIRequest\":{\"MessageHeader\":{\"ProtocolVersion\":\"2.0\",\"MessageClass\":\"Service\",\"MessageCategory\":\"Login\",\"MessageType\":\"Request\",\"ServiceID\":\"498\",\"SaleID\":\"SaleTermA\",\"POIID\":\"POITerm1\"},\"LogoutRequest\":{}}}";

            var hex = string.Join("",

                input.Select(c => String.Format("{0:X2}", Convert.ToInt32(c))));

​

            var output = Encoding.ASCII.GetString(FromHex(hex));

​

            Console.WriteLine($"input: {input}");

            Console.WriteLine($"hex: {hex}");

            Console.WriteLine($"output: {output}");

        }

​

        public static byte[] FromHex(string hex)

        {

            byte[] raw = new byte[hex.Length / 2];

            for (int i = 0; i < raw.Length; i++)

            {

                raw[i] = Convert.ToByte(hex.Substring(i * 2, 2), 16);

            }

            return raw;

        }

}