asp.net - DropDownList は常に Iron Speed の最初のアイテムを返します

Question

Iron Speed で DropDownlist を使用しています。私の問題は、page.ispostback を使用しているにもかかわらず、選択範囲の最初の項目が常に返されることです。これは私のコードです:

Protected Overrides Sub PopulatePaidbyCompPeriodDropDownList(
        _ByVal selectedValue As String,
        _ByVal maxItems As Integer)
    Me.PaidbyCompPeriod.Items.Clear()
    If (Not Page.IsPostBack) Then
        Dim connectionString As String = ConfigurationManager
                .ConnectionStrings("Database").ConnectionString
        Dim conn As New SqlConnection(connectionString)
        conn.Open()
        Dim comm As SqlCommand = New SqlCommand("select  distinct PeriodNumber,
                CONVERT(VARCHAR(10), perioddateto, 23) AS payrolldate,
                YEAR(payrolldate) as yeardate from Payroll order by payrolldate
                desc", conn)
        Dim reader As SqlDataReader = comm.ExecuteReader
        While reader.Read
            PaidbyCompPeriod.Items.Add(New ListItem(MonthName(Month(CDate(
                    reader.GetString(1)))) & " " & Day(CDate(reader.GetString(1)))
                    & ", " & Year(CDate(reader.GetString(1))), reader.GetInt32(0)
                    .ToString))
            DropDownList.Items.Add(New ListItem(reader.GetString(0).ToString,
                    reader.GetString(1).ToString))
        End While
        reader.Close()
        conn.Close()
        PaidByCompDate.Text = CDate(PaidbyCompPeriod.SelectedItem.Text)
                .ToString("MM/dd/yyyy")
    End If
End Sub

この問題を解決するにはどうすればよいですか?