Need help in dropdownlist control - drop-down-menu

Question

I have ridiculous problem with dropdownlist.Through for loop i have inserted dropdown value for Day,Month and Year. But when i am submitting the form after selecting the Day,Month and Year from Dropdownlist,the list item value is reindexing after each submitting.Ex.For Day 1 to 31,Month 1 to 12, year 1970 to 2009.After Submitting the form the list item is becoming double for each dropdownlist.Like in day dropdownlist before submiting it was 1 to 31 after submitting if i check dropdownlist the value will be two times 1 to 31 I mean 1 to 31 again 1 to 31 inside the listitem it happens for each dropdownlist.Here is my code:
aspx:
<asp:DropDownList ID="DropDownDay" runat="server">
<asp:ListItem Text="Select Day"></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownMonth" runat="server">
<asp:ListItem Text="Select Month"></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownYear" runat="server">
<asp:ListItem Text="Select Year"></asp:ListItem>
</asp:DropDownList>
aspx.cs:
//Inserting day in the day dropdown list.
for (int i = 1; i <=31; i++)
{
ListItem item = new ListItem();
item.Value = i.ToString();
DropDownDay.Items.Add(item.Value);
}
//Inserting month in the month dropdown list.
for (int i = 1; i <=12; i++)
{
ListItem item = new ListItem();
item.Value = i.ToString();
DropDownMonth.Items.Add(item.Value);
}
//Inserting year in the year dropdown list.
for (int i = 1970; i <=2009; i++)
{
ListItem item = new ListItem();
item.Value = i.ToString();
DropDownYear.Items.Add(item.Value);
}
string day = DropDownDay.Text;
string month = DropDownMonth.Text;
string year = DropDownYear.Text;
After adding i have set:
DropDownDay.SelectedIndex = 0;
DropDownMonth.SelectedIndex = 0;
DropDownYear.SelectedIndex = 0;
Thanks,
Sumit

Answer 1

you probably are not checking for Page.IsPostBack option
if it is true, then DO NOT REPOPULATE your drop down lists