In this article I will explain with an example, how to solve ASP.Net AJAX ModalPopupExtender Modal Popup Closes on PostBack.
Note: If you want to learn more about ASP.Net ModalPopupExtender and how to use it please refer my article Building Modal Popup using ASP.Net AJAX ModalPopupExtender Control.
 
 

HTML Markup

The HTML Markup consists of following controls
ScriptManager - For enabling ASP.Net AJAX.
Button – For triggering the Modal popup to open.
ModalPopupExtender – For displaying Modal popup.
The ModalPopupExtender has been assigned with following properties.
PopupControlID – The control which will be displayed as Modal popup.
TargetControlID – The control which opens the Modal popup.
CancelControlID – The control which closes the Modal popup.
Panel – For displaying content of the Modal popup.
DropDownList – For capturing user input.
The DropDownList has been assigned with an OnSelectedIndexChanged event handler.
Button – For closing the Modal popup dialog box.
<asp:ScriptManager runat="server">
</asp:ScriptManager>
<asp:Button ID="btnShow" runat="server" Text="Show Modal Popup" />
<!-- ModalPopupExtender -->
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnShow"
    CancelControlID="btnClose" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" Style="display: none">
    <div style="height: 100px">
        Do you like this product?&nbsp;
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged">
    <asp:ListItem Text="Please Select" Value="0"></asp:ListItem>
    <asp:ListItem Text="Yes" Value="1"></asp:ListItem>
    <asp:ListItem Text="No" Value="2"></asp:ListItem>
</asp:DropDownList>
    </div>
    <asp:Button ID="btnClose" runat="server" Text="Close" />
</asp:Panel>
 
 

Solution

You will need to simply re-open the ASP.Net AJAX Modal Popup by calling its Show method, when some control does PostBack, i.e. after the code written for the task to be done, in the last line simply call Show method as shown below.
C#
protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    //Do work
    mp1.Show();
}
 
VB.Net
Protected Sub OnSelectedIndexChanged(sender As Object, e As EventArgs)
    'Do work
    mp1.Show()
End Sub
 
 
 

Demo

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Downloads

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