protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                DropDownList1.DataSource = FileLowResFactory.GetAllFile(929);
               DropDownList1.DataTextField = "FileName";
                DropDownList1.DataValueField = "IdPath";
                DropDownList1.DataBind();

            }
        }


protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            Response.Write(DropDownList1.SelectedValue);
        }

  <asp:DropDownList ID="DropDownList1" runat="server" 
            AutoPostBack="True" 
            onselectedindexchanged="DropDownList1_SelectedIndexChanged1">
        </asp:DropDownList>

DropDownList1.SelectedIndexChanged +=new EventHandler(DropDownList1_SelectedIndexChanged);