I was using modal pop up for both add and edit option on the same page. For this i used ModalPopupExtender of ajaxToolKit .But the

problem which i faced was that when ever i clicked on the edit button.The pop up just came out but i didnt make any changes in the

fields .So i clicked  on the cancel button and the modal pop up just diappeared.The next time when i clicked on the add link the modal

 pop up appeared but it contained the values of the edit link which i clicked before clicking on the add link.

This is the code which i was using in the aspx side

   <ajaxToolkit:ModalPopupExtender ID="mpeCategoryName" runat="server" TargetControlID="lnkBtnAddCategoryName"
            PopupControlID="pnlCategory" DropShadow="true" BackgroundCssClass="modalBackground"
           EnableViewState="false" CancelControlID="btnCancelPopup1">
        </ajaxToolkit:ModalPopupExtender>
        <asp:Panel ID="pnlCategory" runat="server" Width="300px" Height="150px" Style="display: none">
            <table bgcolor="White" border="0">
                <tr>
                 
                    <td>
                        <asp:Button ID="btnCancelPopup1" runat="server" Text="Cancel" OnClick="btnCancelPopup1_Click" />
                    </td>
                </tr>
            </table>
        </asp:Panel>

One of my senior colleague told me to remove CancelControlID="btnCancelPopup1" from here

<ajaxToolkit:ModalPopupExtender ID="mpeCategoryName" runat="server" TargetControlID="lnkBtnAddCategoryName"
            PopupControlID="pnlCategory" DropShadow="true" BackgroundCssClass="modalBackground"
           EnableViewState="false" CancelControlID="btnCancelPopup1">

and use this in the .cs side mpeCategoryName.Hide(); on click event of cancel button and it worked.

  protected void btnCancelPopup1_Click(object sender, EventArgs e)
        {  
            mpeCategoryName.Hide();
         
        }

Hope it will help you.