Combobox onselectedindexchanged event firing from button control

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ASP.NET

Hi,

I have a web form in which two combo box controls and one button.
Firstcombobox data is populated in Page_Load event.

In the firstcombobox.OnsSelectedIndexChanged event i want to populate data in second combo box data based on user selection.

But the firstcombobox.OnSelectedIndexChanged event is not firing on item selection instead it is firing when button is clicked.

Here is the piece of code

  <table>
     <tr>
        <td align="left" >
            <asp:Label ID="labeldate" Text="Starting Date   " runat="server" ></asp:Label>
        </td>
        <td align="left" colspan="2" >
            <asp:TextBox ID="start_date" runat="server"></asp:TextBox>
        </td>
        <ajaxToolkit:CalendarExtender ID="Cal1"  runat="server" TargetControlID="start_date" />
        </tr>

        <br />
        <br />
        <asp:RequiredFieldValidator ID="RequiredFieldValidator1" ControlToValidate="start_date" Text="Starting date is required!" runat="server"></asp:RequiredFieldValidator>

       <tr> <td align="left"><asp:Label ID="label9" Text="Project Code" runat="server" ></asp:Label></td>
       <td colspan="5" ><ajaxToolkit:ComboBox ID="project_code"  runat="server" AutoPostBack="false" Width="312px"   önSelectedIndexChanged="Get_timesheets" > </ajaxToolkit:ComboBox></td>
       </tr>

       <tr> <asp:UpdatePanel ID="updatePanel1"  runat="server" UpdateMode="Conditional">
       <ContentTemplate>
       <td align="left"><asp:Label ID="label10" Text="Timesheet(s) Present" runat="server" ></asp:Label></td>
       <td colspan="5" ><ajaxToolkit:ComboBox ID="Combo_time"  runat="server"   önSelectedIndexChanged="Get_data"  Width="312px"> </ajaxToolkit:ComboBox></td>
       </ContentTemplate>
       </asp:UpdatePanel>

        <td align="center">
            <asp:Button ID="submit" runat="server" Text="OK" Width="60px" OnClick="show_timesheet" />
        </td>
       </tr>
       <asp:RequiredFieldValidator ID="RequiredFieldValidator2" ValidationGroup="btnprj" InitialValue="--Select Project--" ErrorMessage="Please select project code"
       ControlToValidate="project_code" Text="Project code is required!" runat="server"></asp:RequiredFieldValidator>
</table>

Dont' understand why it is doing so.

Any help is highly appreciated

Cheers
Anand