How to assign value to the DropBox text propery? - asp.net

Question

I'm writing a program where user has 3 drop boxes to input date, month and a year. after user selects the values I concatenate them and check for valid By default when a page loads I need to assign a current day, month and year to each drop box accordingly. Then I check validity of the date and pass value to the database.
My problem is that when I assign the values to the text of DropBoxes upon the loading of the page they are becoming permanent. even if the index is changed the values which are passed to the database are the ones which assigned to them when the page is loaded.
I can't actually understand what am I doing wrong:
these are the code samples which I've used:
I populate them with a current date values using folowing code(on the page Load event):
Dim CurYear As Integer = DatePart("yyyy", Now)
Dim CurDate As Integer = DatePart("d", Now)
Dim CurMonth As String = Format(Today.Date, "MMMM")
Dim CurDate2 As Integer = DatePart("d", Now)
Dim CurMonth2 As String = Format(Now, "MM")
Dates.Text = CurDate
Monthe.Text = CurMonth
years.Text = CurYear
Month2.Text = CurMonth2
Dates2.Text = CurDate2
Than I had to synchronize the selected index of 2 the dropboxes which contain numerical value of the month and 2 digit format of the day, to to form the proper string for checking of the date
Protected Sub Months_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles Months.SelectedIndexChanged
Month2.SelectedIndex = Months.SelectedIndex
TextBox3.Text = years.Text & "-" & Monthes.Text & "-" & Dates.Text
End Sub
Protected Sub Dates_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles Dates.SelectedIndexChanged
Dates2.SelectedIndex = Dates.SelectedIndex
TextBox3.Text = years.Text & "-" & Monthes.Text & "-" & Dates.Text
End Sub
And this is front end ASP code:
<asp:DropDownList ID="Dates" runat="server" autopostback="true">
<asp:ListItem></asp:ListItem>
<asp:ListItem>1</asp:ListItem>
...
<asp:ListItem>26</asp:ListItem>
<asp:ListItem>27</asp:ListItem>
<asp:ListItem>28</asp:ListItem>
<asp:ListItem>29</asp:ListItem>
<asp:ListItem>30</asp:ListItem>
<asp:ListItem>31</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="Months" runat="server" autopostback="true" >
<asp:ListItem></asp:ListItem>
<asp:ListItem>January</asp:ListItem>
...
<asp:ListItem>November</asp:ListItem>
<asp:ListItem>December</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="years" runat="server" autopostback="true" >
<asp:ListItem></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="Dates2" runat="server" AutoPostBack="True">
<asp:ListItem></asp:ListItem>
<asp:ListItem>01</asp:ListItem>
<asp:ListItem>02</asp:ListItem>
....
<asp:ListItem>29</asp:ListItem>
<asp:ListItem>30</asp:ListItem>
<asp:ListItem>31</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="Month2" runat="server" AutoPostBack="True">
<asp:ListItem></asp:ListItem>
<asp:ListItem>01</asp:ListItem>
....
<asp:ListItem>11</asp:ListItem>
<asp:ListItem>12</asp:ListItem>
</asp:DropDownList>
Again, if I don't assign the default values to the boxes upon loading of the page it works perfectly. if i do, those values are fixed no mater what you choose
The comparevalidator:
<asp:UpdatePanel ID="UpdatePanel19" runat="server">
<ContentTemplate>
<asp:TextBox ID="TextBox3" ValidationGroup="CompareValidatorDateTest" runat="server"></asp:TextBox>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="Dates" EventName="SelectedIndexChanged" />
<asp:AsyncPostBackTrigger ControlID="Monthes" EventName="SelectedIndexChanged" />
<asp:AsyncPostBackTrigger ControlID="years" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
<asp:CompareValidator ID="CompareValidator3" Display="dynamic" ControlToValidate="TextBox3"
Type="Date" Operator="LessThanEqual" Text="Please enter a valid date" runat="server"
ValidationGroup="CompareValidatorDateTest"

Answer 1

I suppose you didnt use IsPostBack property while binding the dropdown with values on page load.
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not Page.IsPostBack Then
// Bind your dropdown here here
End If