input string was not in correct format asp.net

Question

I have a strange error. I have a page which loads values according to the country value stored in the session. it works fine for all other countries. only for particular country it is returning this error.

protected void populateDDMonth()
{
    int numberOfMonths;
    string monthName;
    string completeCalendarFile = Path.Combine(Request.PhysicalApplicationPath + "admin\\text-files\\", calendarFile);
    TextReader tr = new StreamReader(completeCalendarFile);
    date_classa = tr.ReadLine(); //Get classa end date string
    date_classb = tr.ReadLine(); //Get classb end date string
    DateTime dateConvert_classa = DateTime.Parse(date_classa); 
    DateTime dateConvert_classb = DateTime.Parse(date_classb);
    tr.Close();
    DataTable MonthTable = new DataTable();
    MonthTable.Columns.Add("Month", typeof(string));
    MonthTable.Columns.Add("Date", typeof(string));
    DateTime endMonth = DateTime.Today;
    DDMonth.Items.Clear(); //Clear dropdown in order to re-populate
    string classValue = DDClassType.SelectedValue.ToString();

    if (classValue == "10" || classValue == "12" || classValue == "15")
    {
        endMonth = dateConvert_classa;
    }
    else if (classValue == "9" || classValue == "13" || classValue == "16")
    {
        endMonth = dateConvert_classb;
    }
    if (endMonth.Year > DateTime.Today.Year)
    {
        numberOfMonths = (endMonth.Month + 12) - DateTime.Today.Month;
    }
    else
    {
        numberOfMonths = endMonth.Month - DateTime.Today.Month;
    }
    for (int i = 0; i < numberOfMonths + 1; i++)
    {
        monthName = DateTime.Now.AddMonths(i).ToString("MMM"); //Display month as 3 letter string
        DataRow MonthRow = MonthTable.NewRow();
        MonthRow[0] = monthName.ToString() + " " + DateTime.Now.AddMonths(i).Year.ToString();
        MonthRow[1] = DateTime.Now.AddMonths(i).ToString();
        DDMonth.Items.Add(new ListItem(monthName.ToString() + " " + DateTime.Now.AddMonths(i).Year.ToString(),
        DateTime.Now.AddMonths(i).ToString()));
    }
    if (Session["selectedMonth"] != null)
    {
        DDMonth.SelectedValue = Session["selectedMonth"].ToString();
    }

    if (!Page.IsPostBack)
    {

        DateTime startingDate = DateTime.Parse(DDMonth.SelectedValue.ToString());
        LbCalendarCurrentMonth.Text = startingDate.ToString("MMMM");

    }
}

Answer 1

This is a common problem with int casts and conversions within the International cultures:

http://support.microsoft.com/kb/942460

Answer 2

Whatever value is selected (if any) in DDMonth drop down, is not a valid date.

So the following code should work without error:

DateTime startingDate;
if (DateTime.TryParse(DDMonth.SelectedValue.ToString(), out startingDate)
{
   LbCalendarCurrentMonth.Text = startingDate.ToString("MMMM");
}