title: DropDownList SelectedIndexChanged event fires twice on few Dropdowns
body: <p>I have very simple aspx page &amp; code:</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;Untitled Page&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;form id="form1" runat="server"&gt;
      &lt;div&gt;
  
      &lt;/div&gt;
      &lt;asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" EnableViewState="false"&gt;
          &lt;asp:ListItem&gt;Item1&lt;/asp:ListItem&gt;
          &lt;asp:ListItem&gt;Item2&lt;/asp:ListItem&gt;
          &lt;asp:ListItem&gt;Item3&lt;/asp:ListItem&gt;
      &lt;/asp:DropDownList&gt;
      &lt;asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" EnableViewState="false"&gt;
          &lt;asp:ListItem&gt;Item1&lt;/asp:ListItem&gt;
          &lt;asp:ListItem&gt;Item2&lt;/asp:ListItem&gt;
          &lt;asp:ListItem&gt;Item3&lt;/asp:ListItem&gt;    
      &lt;/asp:DropDownList&gt;
      &lt;asp:DropDownList ID="DropDownList3" runat="server"  AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" EnableViewState="false"&gt;
          &lt;asp:ListItem&gt;Item1&lt;/asp:ListItem&gt;
          &lt;asp:ListItem&gt;Item2&lt;/asp:ListItem&gt;
          &lt;asp:ListItem&gt;Item3&lt;/asp:ListItem&gt;    
      &lt;/asp:DropDownList&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  
  
  using System;
  using System.Configuration;
  using System.Data;
  using System.Linq;
  using System.Web;
  using System.Web.Security;
  using System.Web.UI;
  using System.Web.UI.HtmlControls;
  using System.Web.UI.WebControls;
  using System.Web.UI.WebControls.WebParts;
  using System.Xml.Linq;
  
  public partial class _Default : System.Web.UI.Page 
  {
      protected void Page_Load(object sender, EventArgs e)
      {
  
      }
  
      protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
      {
          Response.Write("DropDownList1_SelectedIndexChanged called! &lt;br /&gt;");
      }
  }
  </code></pre>
  
  <p>EnableViewState="false" for all DropDown.</p>
  
  <p>Could anyone explain me why DropDownList1_SelectedIndexChanged event executes twice after first dropdown value changed, then change value in second or third dropdown.</p>
  
  <p>Thanks! </p>