Add and remove columns from a Tablix in rdlc report

Question

I want to create a report with dynamic number of columns. I have used the solutions of these two following links to load and modify my rdlc report file.

1. http://dobrzanski.net/2009/02/06/how-to-dynamically-add-new-columns-to-report-created-with-reporting-services/
2. Why does C# XmlDocument.LoadXml(string) fail when an XML header is included?

but the problem is that when i use the Save method of xmlDocument to save the report anything happens.

what is my mistake???