does not contain a definition for 'Add'

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ASP.NET

C#3.5

'System.Web.UI.MobileControls.List' does not contain a definition for 'Add' and no extension method 'Add' accepting a first argument of type 'System.Web.UI.MobileControls.List' could be found (are you missing a using directive or an assembly reference?)
My code attached here.

protected void Button2_Click(object sender, EventArgs e) {

    var l = new List();
    sc.Open();
    SqlCommand cmd = new SqlCommand("select Fliono from BOND_REG where status='A'", sc);
    SqlDataReader SDR = cmd.ExecuteReader();


    while (SDR.Read() == true)
    {
        var item1 = SDR.GetValue(0).ToString();
        l.Add(item1);

    }
    fileexport(l);
    updatestaus(l);
    SDR.Close();
    sc.Close();

}

public void updatestaus(List F)
{
    StringBuilder sb = new StringBuilder();

    for (int i = 0; i < F.Items.Count - 2; i++)
    {
        sb.Append(F.Items[i].ToString() + ",");
    }
    sb.Append(F.Items[F.Items.Count - 1].ToString());


    sc.Close();
    sc.Open();
    SqlCommand cmd = new SqlCommand("update BOND_REG set status='P' where fliono IN ('" + sb.ToString() + "')", sc);
    cmd.ExecuteNonQuery();
    sc.Close();
}

public void fileexport(List F)
{
    // SDR.Close(); 
    StringBuilder sb = new StringBuilder();

    for (int i = 0; i < F.Items.Count - 2; i++)
    {
        sb.Append(F.Items[i].ToString() + ",");
    }
    sb.Append(F.Items[F.Items.Count - 1].ToString());


    SqlCommand cmd1 = new SqlCommand("select * from BOND_REG where status='A' and Fliono IN ('" + sb.ToString() + "')", sc);
    SqlDataAdapter da = new SqlDataAdapter();
    da.SelectCommand = cmd1;
    DataTable datatable = new DataTable();
    da.Fill(datatable);
    ReportDocument crystalReport = new ReportDocument();
    crystalReport.Load(Server.MapPath("~/bond.rpt"));
    crystalReport.SetDataSource(datatable);
    ExportOptions rptExportOption;
    DiskFileDestinationOptions rptFileDestOption = new DiskFileDestinationOptions();
    PdfRtfWordFormatOptions rptFormatOption = new PdfRtfWordFormatOptions();
    string reportFileName = @"C:\pdfd\ st1.pdf";
    rptFileDestOption.DiskFileName = reportFileName;
    rptExportOption = crystalReport.ExportOptions;
    {
        rptExportOption.ExportDestinationType = ExportDestinationType.DiskFile;
        rptExportOption.ExportFormatType = ExportFormatType.PortableDocFormat;
        rptExportOption.ExportDestinationOptions = rptFileDestOption;
        rptExportOption.ExportFormatOptions = rptFormatOption;
    }

    crystalReport.Export();
}