How to create unique id with mysql and c#?

Question

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

using MySql.Data.MySqlClient;
using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SistemPakar
{
        class MySQLService : MySQLConn
        {
            private String strCon = "";
            private MySqlConnection sConn;
            private MySqlCommand sComm;
            private MySqlDataAdapter dtAdp;
            public String Query;

            public MySQLService()
            {
                strCon = Constr();
                sConn = new MySqlConnection(strCon);
                sComm = new MySqlCommand();
                dtAdp = new MySqlDataAdapter();
            }

            public String Constr()
            {
                String data;
                data = "SERVER=localhost; DATABASE=csharp_sistempakar; UID=root; PWD=;";
                return data;
            }

            public void BukaKoneksi()
            {
                if (sConn.State == ConnectionState.Closed)
                {
                    try
                    {
                        sConn.Open();
                    }
                    catch (Exception)
                    { }
                }
            }

            public void TutupKoneksi()
            {
                sConn.Close();
            }

            public override int ExecNonQuery(String Query)
            {
                int retVal = -1;

                try
                {
                    BukaKoneksi();
                    sComm.Connection = sConn;
                    sComm.CommandText = Query;
                    retVal = sComm.ExecuteNonQuery();
                }
                catch (Exception) { }
                finally
                {
                    TutupKoneksi();
                }
                return retVal;
            }

            public override DataTable ExecQuery(String Query)
            {
                DataTable retVal = new DataTable();
                try
                {
                    BukaKoneksi();
                    sComm.Connection = sConn;
                    sComm.CommandText = Query;
                    dtAdp.SelectCommand = sComm;
                    dtAdp.Fill(retVal);
                }
                catch (Exception) { }
                finally
                {
                    TutupKoneksi();
                }
                return retVal;
            }
        }
}

Class MySQLConn.cs

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

namespace SistemPakar
{
    class Srvc
    {
        //Membuat class untuk menampilkan id
        private String id;

        public Srvc()
        {
            id = "";
        }

        public String ID
        {
            set { id = value; }
            get { return id; }
        }
    }
}

Class Srv.cs

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

namespace SistemPakar
{
    class Srvc
    {
        //Membuat class untuk menampilkan id
        private String id;

        public Srvc()
        {
            id = "";
        }

        public String ID
        {
            set { id = value; }
            get { return id; }
        }
    }
}

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<String>("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;
        }
    }
}