listbox retains posted value even after being databound

please note problem described next roughly accurate opposing common problem "my control shows same value each page loads. i know function spoken generally desirable.

i have listbox being databound page bucket eventuality even postback.

when eventuality handler selectedindex altered hit, control somehow posted value even nonetheless already organization again should have selectedindex during point.

does anyone know possible.

edit:

to denote selectedindex indeed reset emanate form following rudimentary markup:

    <label for="textbox1">original posted value: </label>
    <asp:textbox runat="server" id="textbox1" />
    <asp:dropdownlist runat="server" id="dropdown" onselectedindexchanged="dropdown_selectedindexchanged" autopostback="true" />
    <label for="textbox2">value during indicate handler hit: </label>
    <asp:textbox runat="server" id="textbox2" />

with following formula .cs

        stable vacant page_load(object sender, eventargs e)
    {
        textbox1.text = dropdown.selectedindex.tostring();
        dropdown.datasource = new string[] { "none", "a", "b", "c" };
        dropdown.databind();
    }
    stable vacant dropdown_selectedindexchanged(object sender, eventargs e)
    {
        textbox2.text = dropdown.selectedindex.tostring();
    }

notice value second textbox alway 0.