asp.net - DropDownList 总是返回 Iron Speed 中的第一项

Question

我在 Iron Speed 中使用 DropDownlist。我的问题是它总是返回选择中的第一项，即使我使用 page.ispostback。这是我的代码：

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

我该如何解决这个问题？