SelectedValue of the DropDownList has not changed. You could use Request.Form("__EVENTTARGET") in the Page_Load to solve this problem.
Here is the example:
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Default.aspx.vb" Inherits="TestDropDown._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true">
<asp:ListItem Text="---" Value="---"></asp:ListItem>
<asp:ListItem Text="aa" Value="aa"></asp:ListItem>
<asp:ListItem Text="bb" Value="aa"></asp:ListItem>
<asp:ListItem Text="cc" Value="cc"></asp:ListItem>
</asp:DropDownList>
</div>
</form>
</body>
</html>
Partial Public Class _Default
Inherits System.Web.UI.Page
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Me.Request.Form("__EVENTTARGET") = DropDownList1.UniqueID Then
Response.Write(DropDownList1.SelectedValue)
End If
End Sub
Private Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
Response.Write(DateTime.Now.ToString())
End Sub
End Class
