จะทำการเพิ่มให้รหัสโดยเพิ่มรหัสลุกค้าขึ้นก่อน ตามด้วยเลขรัน ขึ้นErrorค่ะ

Private Sub generate()
        Dim sqlmem As String = ""
        Dim tmp As String = ""      '------------เก็บรหัสลูกค้าชั่วคราว
        Dim First As Integer = 0
        Dim Last As Integer = 0
        sqlmem = "select TOP 1 ISNID from ISN where (CarType = '" & CStr(ComboBox1.SelectedValue) & "')"
        sqlmem &= " order by ISNID desc"
        With conn
            If .State = ConnectionState.Open Then .Close()
            .ConnectionString = state
        End With
        Try
            With dc
                .CommandType = CommandType.Text
                .CommandText = sqlmem
                .Connection = conn
                drr = .ExecuteReader()
                drr.Read()

                tmp = CStr(drr.Item("ISNID"))

                First = CInt(Microsoft.VisualBasic.Left(tmp, 2))
                Last = CInt(Microsoft.VisualBasic.Right(tmp, 4))

                Last = Last + 1
                TextBox1.Text = First.ToString("00") & Last.ToString("0000")

            End With

        Catch
            TextBox1.Text = CStr(ComboBox1.SelectedValue) & "000"

        End Try

        drr.Close()

    End Sub