Asp Modal Popup:How to prevent from closing

[arvee2003]

In an aspx page I have an user control which in turn opens a modal popup which is
another user control.

In the usercontrol in modalPopup I have a check box with a checkedChanged event.

When the checkbox is checked, I check/uncheck all items in a check box list in the user control.

However when the event is completed it closes the modal popup. How do I avoid the modal
popup from being closed. I do not have a modalpopupextender property in user control

I tried using return statement, it does not work.

ANy input is appreciated.

Thanks
Suresh

protected void CheckBoxSelectAll_CheckedChanged(object sender, EventArgs e)
    {
        Int32 indexCount = 0, totalCount = 0;
        totalCount = Convert.ToInt32(cblLevelList.Items.Count);
        Control ctrl = (Control)FindControl("ModalPopupExtenderLevel");
        if (CheckBoxSelectAll.Checked)
        {
            for (indexCount = 0; indexCount < totalCount; indexCount++)
            {
                cblLevelList.Items[indexCount].Selected = true;
            }
        }
        else
        {
            for (indexCount = 0; indexCount < totalCount; indexCount++)
            {
                cblLevelList.Items[indexCount].Selected = false;

            }
        }
         
        return;
        
    }

<asp:Panel ID="PanelLevelHeader" runat="server" style="height:250px;position:absolute;width:320px;text-align:center;" HorizontalAlign="Center">
        <asp:CheckBox  id="CheckBoxSelectAll" runat="server" Text="Select All" 
                 ToolTip="Click to select all Level"
               Style="text-align:center;font-family:Arial,Helvetica;font-style:normal;font-size:9pt;" 
              OnCheckChanged="CheckBoxSelectAll_CheckedChanged" CausesValidation="True" AutoPostBack="True">
         </asp:CheckBox>
         <hr />
         </asp:Panel>
          <br /> 
         <asp:Panel ID="PanelLevelDetail" runat="server" style="height:200px;width:320px;margin-top:10px;background-color:#fff; "       ScrollBars="Auto">    
             
            <asp:CheckBoxList ID="cblLevelList" runat="server" style="height:40px;text-align:left;font-family:Arial,Verdana;font-size:9pt;"  
                    DataSourceID="ObjectDataSourceLevel" DataTextField="rv_meaning" DataValueField="rv_low_value" Height="40px" >      
                   </asp:CheckBoxList>
                   <asp:ObjectDataSource ID="ObjectDataSourceLevel" runat="server"  
                   TypeName="locator_reference_codes" SelectMethod="GetMultiTrainingLevels" >    
            </asp:ObjectDataSource>
         </asp:Panel>