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I am using Excel Interop to open an xlsx file and save that as a pdf document. Upon invoking the 'ExportAsFixedFileFormat' method a dialog titled "Publishing" is displayed to indicate the progress. How can I suppress or hide this dialog? I have seen a few similar questions on other forums without a satisfying solution, but hopefully someone has solved this since then.

Code:

Application application = new Application();
application.DisplayAlerts = false; // <- No effect
application.Visible = false; // <- No effect
application.ScreenUpdating = false; // <- No effect
application.UserControl = false; // <- No effect
application.Workbooks.Open(path, Type.Missing, true);
application.DisplayDocumentActionTaskPane = false; // <- No effect
application.Worksheets[1].ExportAsFixedFormat(XlFixedFormatType.xlTypePDF, path);

Perhaps I am not answering your question, but I'll try to be helpful. As far as I know, it's not recommended to use Excel Interop in back-end processing. You encountered one of its issues, but there are some others. There are 3rd party alternatives for interacting with Excel formats, some of them are open source and some are commercial. I would strongly recommend taking a look at one of them:
ClosedXML and NPOI. Both are open source and free.

How about processing it on a BackgroundWorker. This will not throw COMEXCEPTION.
I tried this code on a button click and it works without displaying the progress dialog.

var worker = new BackgroundWorker();
worker.DoWork += (o, args) =>

 var path = @"D:sample.xlsx";
 var application = new Microsoft.Office.Interop.Excel.Application();
 application.Workbooks.Open(path,
 Type.Missing, true);
 application.Worksheets[1].ExportAsFixedFormat(XlFixedFormatType.xlTypePDF, path);
;
worker.RunWorkerAsync();

You can use pedamorf library ,pedamorf is a server that converts documents, images, urls, html and text to PDF. It relies heavily upon the excellent open-source programs wkhtml2pdf, Libre Office and iTextSharp. A client library is provided and can be used from a .NET application to utilize the conversion services provided by pedamorf.

More info is here

By far the better API to handle (create, read, export to PDF/html, etc...) XLS/XLSX files that I know is FlexCel. It's really simple, incredibly fast and full writen in C#. Don't require Excel instalation.

You'll find it here : http://www.tmssoftware.com/site/flexcelnet.asp (get the trial version with a lot of examples with source code)

If you have budget in your project to acquire the licence, I realy reccomend.