Convert ascii code to hex string in c#

using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Diagnostics;
class Convert
{

    public void DoConversion()
    {
        string str = "test";
        byte[] byteArray = null;
        System.Text.StringBuilder hexNumbers = new System.Text.StringBuilder();
        byteArray = System.Text.ASCIIEncoding.ASCII.GetBytes(str);
        for (int i = 0; i <= byteArray.Length - 1; i++) {
            hexNumbers.Append(byteArray[i].ToString("x"));
        }
        Interaction.MsgBox(hexNumbers.ToString());
        // Out  put will be 74657374

        // The following code will reverse the operation and verify the output:
        string st = hexNumbers.ToString();
        string com = "";
        for (x = 0; x <= st.Length - 1; x += 2) {
            com += Strings.ChrW(Convert.ToInt32("&H" + st.Substring(x, 2)));
        }
        Interaction.MsgBox(com);
    }
}