How to create unique id with mysql and c#?

I try every video in youtube about unique id with C#. But, every video using SQL Server.

I create database id using varchar for input A0001, and try to covert code for MySQL database. But, not working.

This is detail code:

Class MySQLService.cs

Class MySQLConn.cs

Class Srv.cs

Class Service.cs

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace SistemPakar
{
class Service : Srvc
{
MySQLService mysql;
DataTable dt;
public Service()
{
mysql = new MySQLService();
dt = new DataTable();
}
public String IDKostum()
{
String kode = "";
int idx = 0;
mysql.Query = "Select isnull(MAX(Right(code_hypothesis,4)),0) as jml From hypothesis";
dt = mysql.ExecQuery(mysql.Query);
if (dt.Rows.Count > 0)
{
foreach (DataRow dtRow in dt.Rows)
{
idx = Convert.ToInt32(dtRow.Field("jml"));
}
}
if (idx >= 0 && idx <= 8)
{
kode = "H" + "000" + Convert.ToString(idx + 1);
}
else if (idx >= 9 && idx <= 98)
{
kode = "H" + "00" + Convert.ToString(idx + 1);
}
else if (idx >= 99 && idx <= 998)
{
kode = "H" + "0" + Convert.ToString(idx + 1);
}
else if (idx >= 999 && idx <= 9998)
{
kode = "H" + Convert.ToString(idx + 1);
}
return kode;
}
}
}

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