DropDownList in ASP.net pages has OnSelectedIndexChanged fire twice - asp.net

html
<asp:DropDownList ID="ddlOffice" runat="server" Rows="10" Width="300px" AutoPostBack="true" OnSelectedIndexChanged="ddlOffice_SelectedIndexChanged" EnableViewState="true" />
Even though according to MSDN I am doing everything correctly. the SelectedIndexChanged still fires twice. Any ideas how this can fixed?

One possible reason: If you have a also registered the event handler in codebehind(f.e. via Handles clause in VB.NET or += in C#) it will cause the event to be fired twice.
Then you just have to remove one of it, for example on aspx:
<asp:DropDownList ID="ddlOffice"
runat="server" Rows="10" Width="300px"
AutoPostBack="true"
EnableViewState="true" />

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<asp:Login ID="lgLogin" runat="server" OnLoggingIn="lgLogin_OnLoggingIn" />
and on normal page I have:
<asp:DropDownList runat="server" ID="ddl" AutoPostBack="true" />
If I do login action first, there is no problem.
If I change DDL items, it causes postback. And after that, login does't work. It causes postback, but doesn't call lgLogin_OnLoggingIn method.

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There is no way to turn off the postbacks to the server but you can use updatepanel control which can help you some what.
According to PLBlum at the ASP.NET forums:
The Calendar control included with ASP.NET uses postbacks only. If you
added Microsoft ASP.NET AJAX to the page and put the calendar into an
UpdatePanel, it can reduce the appearance of postbacks by using
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Many people have created replacements to the Calendar control that
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<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
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</ContentTemplate>
</asp:UpdatePanel>
notice that you should use UseSubmitBehavior="False" with buttons also to stop postback for buttons in the same updatePanel
It works fine
I think you can use "CalendarExtender" from AjaxToolkit
<asp:TextBox ID="txtCalendarExtender" runat="server"></asp:TextBox>
<cc3:CalendarExtender ID="Calendar1" PopupButtonID="imgPopup" runat="server" TargetControlID="txtCalendarExtender" Format="dd/MM/yyyy">
</cc3:CalendarExtender>
Remember to add this code at the top of your page:
<%# Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc3" %>

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It has to do with the event-chain in ASP.Net, this MSDN page helps to understand it.
The .Net framework resolves the ViewState between the Init- and the Load-event. So if the control is not there after the Init-event, it's ViewState will never be loaded and also it's events will never be fired.
This can get quite tricky at times, especially if one dynamically loaded control should affect the loading of another one...

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