ASP.NET Dropdownlist SelectedIndexChanged fired again after rasing exception - asp.net

Question

Here's my sample code.
My scenarios is
1. Change dropdownlist item it will fire SelectedIndexChanged event.
2. During SelectedIndexChanged event, raise the exception.
3. Popup error alert message
4. Click the button
5. SelectedIndexChanged fired again.
I'm really curious to know what's going on this procedure?
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<script type="text/javascript">
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(PageRequestManager_OnEndRequest);
function PageRequestManager_OnEndRequest(sender, args) {
//debugger;
var alertMessage = "Application exception has been occurred.\n\nMessage: ";
if (args.get_error() && args.get_error().httpStatusCode == '500') {
var errorMessage = args.get_error().message
var findIndex = 0;
args.set_errorHandled(true);
if ((findIndex = errorMessage.indexOf(":")) > -1) {
errorMessage = errorMessage.substr(findIndex + 2);
}
alert(alertMessage + errorMessage);
}
};
</script>
<div>
<asp:UpdatePanel runat="Server" UpdateMode="Conditional" ID="UpdatePanel1">
<ContentTemplate>
<asp:DropDownList ID="DropDownListState" runat="server" OnSelectedIndexChanged="DropDownListState_SelectedIndexChanged" AutoPostBack="true">
<asp:ListItem Text="Michigan" Value="07"></asp:ListItem>
<asp:ListItem Text="Mississippi" Value="08"></asp:ListItem>
<asp:ListItem Text="Nevada" Value="09"></asp:ListItem>
</asp:DropDownList>
<asp:Button runat="server" ID="Button" OnClick="Button_Click" Text="Click Here" />
</ContentTemplate>
</asp:UpdatePanel>
</div>
</form>
codebehind:
protected void DropDownListState_SelectedIndexChanged(object sender, EventArgs e)
{
int stateNo = 0;
string stateName = "California";
stateNo = Convert.ToInt32(stateName);
}
protected void Button_Click(object sender, EventArgs e)
{
string ddd = "";
}

Answer 1

It happens because error in event DropDownListState_SelectedIndexChanged ends the request to the server in abnormal way. That's why not all properties and states are properly propagated. So when you hit the button it will think that dropdownlist value on the server is different that the one provided by the browser and will fire DropDownListState_SelectedIndexChanged again. If you comment out the error all will work as expected.