auto generate alphanumeruc number

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VB

Good Day Experts!,

i would like to ask if anyone can help me with my code. i want to display a auto-generating alphanumeric numbers like this:

OR06161400001

which is in my code is something like this:

Dim con As New SqlConnection(ConnectString())
       Dim cd As New SqlCommand
       Dim lrd As SqlDataReader
       Try
           con.Open()
           cd.Connection = con
           cd.CommandText = "SELECT count(ornumber) as co FROM BlueRoomTransactions"
           lrd = cd.ExecuteReader()
           While lrd.Read()
               ornum = Convert.ToString(lrd("co"))
           End While
       Catch ex As Exception
       Finally
           con.Close()
       End Try

       ornum = ornum + 1

txtORNumber.Text = "OR" + Date.Today.ToString("MMddyy") + "" + ornum.ToString.PadLeft(5, "0")

what i want is to display the first alphabet "A" after the date.then if it reach 100,Letter "A" becomes "B".

i tried using this code but all i get is the last alphabet.

Dim iChar As Integer

           For iChar = Asc("a") To Asc("z")

               MsgBox(Chr(iChar))

           Next iChar

Thanks.