ASP.NET

<script language="javascript" type="text/javascript">
    function StartProgressBar() {
        var myExtender = $find('ProgressBarModalPopupExtender');
        myExtender.show();
        return true;
    }
    function StopProgressBar() {
        var myExtender = $find('ProgressBarModalPopupExtender');
        myExtender.hide();
        return true;
    }

    function Valid() {
        if (Page_ClientValidate("Required") == true) {
            StartProgressBar()
        }
        else {
            alert("Please fill the Required fields");
        }

    }
</script>

<asp:Button ID="btnSubmit" runat="server" Text="Submit" CssClass="Button" OnClientClick="Valid()" />

 <div>
             <cc1:ModalPopupExtender ID="ProgressBarModalPopupExtender" runat="server" BackgroundCssClass="ModalBackground" behaviorID="ProgressBarModalPopupExtender"  TargetControlID="hiddenField" PopupControlID="Panel1" />
             <asp:Panel ID="Panel1" runat="server" Style="display: none; background-color: #C0C0C0;">

                    <asp:Image ID="Image3" ImageUrl="~/Images/712Blue.GIF" runat="server" AlternateText="" />

                </asp:Panel>

                <asp:HiddenField ID="hiddenField" runat="server" />
 </div>

Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSubmit.Click
    If Page.IsValid Then
     Thread.Sleep(300)
     ProgressBarModalPopupExtender.Hide()
     ClearAll()
    End If
 End Sub

if ddl="abc" and fileuplaod1.HasFile Then
   	UploadFile
   	Bool=true
Else if ddl="abc" and FileUpload1.HasFile=false Then
   	Alert asking for fileupload
	Bool=false
Else
	Bool=True

If Bool=True then 
DataBind to Gridview

Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed.

<asp:UpdatePanel ID="Upnl1" runat="server">
<Triggers>
 <asp:PostBackTrigger ControlID="btnAddLineItems" />
</Triggers>
 <ContentTemplate>
    <div>
    </div>
    <asp:panel>
    <table>
        txt1
        txt2
        ddl1
        ddl2
        File upload
        Button Add new Items
    </table>
    </asp:panel>

    <asp:updatepanel>
        <ContentTemplate>
        <Gridview>
        <Boundfield 1=txt1>
        <Boundfield 2=txt2>
        <Boundfield 3=ddl1>
        <Boundfield 4=ddl3>
        <Templatefield Link button="Download">
        </Gridview>
        </ContentTemplate>
    <asp:updatepanel>

 </ContentTemplate>
</asp:UpdatePanel>

Dim filePath As String = CType(sender, LinkButton).CommandArgument
       Response.ContentType = ContentType
       Response.AppendHeader("Content-Disposition", ("attachment; filename=" + Path.GetFileName(filePath)))
       Response.WriteFile(filePath)
       Response.End()