title: Dropdownlist selectedindexchanged event is not firing
body: <p>Simply I have a Dropdownlist with RequiredFieldValidatior in UpdatePanel on a page,
  I have enabled autopostback for the dropdownlist.</p>
  
  <p>The problem is that Dropdownlist selectedindex event is not firing.
  This unexpected behavior happens when I validate the page and ant error occurs.</p>
  
  <p>I searched a lot but unable to find the solution</p>
  
  <p>my code is as follows:</p>
  
  <pre><code>&lt;%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %&gt;
  
  &lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
  &lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
  &lt;head runat="server"&gt;
      &lt;title&gt;&lt;/title&gt;
      &lt;script type="text/javascript"&gt;
          function ValidateMe() {
              if (Page_ClientValidate("vgOption")) {
                  alert("valid");
              }
  
              return false;
          }
      &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;form id="form1" runat="server"&gt;
      &lt;asp:ScriptManager ID="smMain" runat="server"&gt;
      &lt;/asp:ScriptManager&gt;
      &lt;asp:UpdatePanel ID="pnlMain" runat="server" ChildrenAsTriggers="true"&gt;
          &lt;ContentTemplate&gt;
              &lt;table border="1" cellpadding="5" cellspacing="0"&gt;
                  &lt;tr&gt;
                      &lt;td&gt;
                          Option:
                      &lt;/td&gt;
                      &lt;td&gt;
                          &lt;asp:DropDownList runat="server" AutoPostBack="true" ID="Opt" OnSelectedIndexChanged="Opt_SelectedIndexChanged" ValidationGroup="vgOption"&gt;
                              &lt;asp:ListItem Text="--Select Option--" Value="0" /&gt;
                              &lt;asp:ListItem Text="Upload" /&gt;
                              &lt;asp:ListItem Text="Download" /&gt;
                          &lt;/asp:DropDownList&gt;
                          &lt;asp:RequiredFieldValidator ID="rfv" runat="server" ControlToValidate="Opt" Display="None" InitialValue="0" ValidationGroup="vgOption" ErrorMessage="Please select an option"&gt;&lt;/asp:RequiredFieldValidator&gt;
                      &lt;/td&gt;
                  &lt;/tr&gt;
                  &lt;tr&gt;
                      &lt;td&gt;
                          Postback:
                      &lt;/td&gt;
                      &lt;td&gt;
                          &lt;asp:Label Text="" ID="lblMessage" runat="server" /&gt;
                      &lt;/td&gt;
                  &lt;/tr&gt;
                  &lt;tr&gt;
                      &lt;td&gt;
                      &lt;/td&gt;
                      &lt;td&gt;
                          &lt;input type="button" onclick="return ValidateMe();" value="Test" title="Test" /&gt;
                          &lt;asp:ValidationSummary ValidationGroup="vgOption" runat="server" ShowMessageBox="true" ShowSummary="false" DisplayMode="List" /&gt;
                      &lt;/td&gt;
                  &lt;/tr&gt;
              &lt;/table&gt;
          &lt;/ContentTemplate&gt;
      &lt;/asp:UpdatePanel&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>Codebehind:</p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Web;
  using System.Web.UI;
  using System.Web.UI.WebControls;
  
  public partial class _Default : System.Web.UI.Page
  {
      protected void Page_Load(object sender, EventArgs e)
      {
  
      }
  
      protected void Opt_SelectedIndexChanged(object sender, EventArgs e)
      {
          lblMessage.Text = "Autopostback: " + DateTime.Now.ToString();
      }
  }
  </code></pre>
  
  <p>Steps to repopulate the issue:<br/>
  1. Click first option in dropdown<br/>
  2. click on submit button<br/>
  3. change dropdownlist value (this should fire selectedindex changed event, but it doesn't)</p>
  
  <p><strong>PS:</strong> I do not want to postback to happen when the submit button is clicked that is why I added <code>&lt;input&gt;</code> instead of asp.net button,<br/>
  even if I add asp.net button it doesnt work</p>