Dropdownlist Selected value is not working - asp.net

Question

I have 2 dropdowns ddl1 and ddl2. When im setting selected value for ddl1 it works,but after setting selected value for ddl2 also changes the value of ddl1.I dont know how but its doing so.
aspx
<asp:DropDownList ID="ddlPickup" runat="server" CssClass="tb_date"
>
</asp:DropDownList>
Dropoff:
<asp:DropDownList ID="ddlDropoff" runat="server" CssClass="tb_date" >
</asp:DropDownList>
<asp:RadioButtonList ID="radiobuttonlist1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="radiobuttonlist1_SelectedIndexChanged"
RepeatDirection="Horizontal">
<asp:ListItem Value="1">Day or Longer</asp:ListItem>
<asp:ListItem Value="2">1/2 Day</asp:ListItem>
<asp:ListItem Value="3">Hourly</asp:ListItem>
</asp:RadioButtonList>
code behind
protected void radiobuttonlist1_selectedindexchange(object sender, EventArgs e)
{
if(radiobuttonlist1.SelectedValue=="1")
{
Setddl();
}
}
public void Setddl()
{
if (ddlPickup.Items.Count > 0)
{
ddlPickup.Items.Clear();
}
if (ddlDropoff.Items.Count > 0)
{
ddlDropoff.Items.Clear();
}
ListItem li = new ListItem();
DateTime date = Convert.ToDateTime(StartDate.Text);
int day = Convert.ToInt32(date.DayOfWeek);
long itemid = Convert.ToInt64(ddlRentalItem.SelectedValue);
if (itemid != 0)
{
var dropoff = _objRitems.GetHourlyHourByDay(itemid, day);
if (dropoff.Count > 0)
{
int stimeh = Convert.ToDateTime(dropoff[0].OpenTime).Hour;
int etimeh = Convert.ToDateTime(dropoff[0].CloseTime).Hour;
DateTime dt = Convert.ToDateTime(dropoff[0].OpenTime);
for (int i = 0; i <= (etimeh - stimeh) * 2; i++)
{
string time = string.Format("{0:t}", dt);
li = new ListItem(time, time);
ddlPickup.Items.Add(li);
ddlDropoff.Items.Add(li);
dt = dt.AddMinutes(30);
}
ddlPickup.DataBind();
ddlDropoff.DataBind();
ddlPickup.SelectedValue=Request.QueryString["droptime"].ToString();
ddlDropoff.SelectedValue=Request.QueryString["droptime"].ToString();
//as soon as ddlDropoff SelectedValue assigns ddlPickup.SelectedValue changes to ddlDropoff.SelectedValue..Very Weird!!!
}
else
{
}
}
else
{
}
}

Answer 1

you need to write your dropdown code in below if condition.
if(!isPostBack)
{
}

Answer 2

Forget about ddlPickup_DataBound. Rely on the ViewState implicit in dropdown lists to repopulate the selected value. The ondatabound methods are unnecessary in this case.
Also you only need to bind the dropdowns once so on Page.IsPostBack - like the other poster has said
This would be the more standard way to deal with drop downs i.e.
Bind once
SelectedValue persists in after postbacks. No need for special work here
The way you are doing it - the databound will fire on each page load (after each dropdown if autopostback is true). The both dropdowns will repopulate every time - depending what yor have got in your Request collection. This isn't what you intend i assume and would account for the strange behaviour.
EDIT:
Just to be really clear this is the pattern that should be followed
Markup
<asp:DropDownList ID="ddlPickup" runat="server" ></asp:DropDownList>
<asp:DropDownList ID="ddlDropoff" runat="server" OnSelectedIndexChanged="selectedIndexEvent"></asp:DropDownList>
Code behind
protected void Page_Load(object sender, EventArgs e)
{
if(!Page.IsPostBack)
{
BindDropDownLists();
}
}
protected void ddl2_selectedindexchange(object sender, EventArgs e)
{
//.. legit - do whatever is needed but don't reset selected value
}
protected void radiobuttonlist1_selectedindexchange(object sender, EventArgs e) {
{
BindDropDownLists();
SetInitialDropDownValues();
}
No ondatanound is needed