Error opening an HTML file in rich text box

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C#

WPF

I am developing an application in WPF. I want some *.html files to open in the rich text box.
I am getting the following error: Cannot access C drive.
I am using the following code.

richTextBox_LessonView.Document = new FlowDocument();
                TextRange documentTextRange = new TextRange(
                    richTextBox_LessonView.Document.ContentStart, richTextBox_LessonView.Document.ContentEnd);

                using (FileStream fs = File.Open(folderloc + display_lesson, FileMode.Open))

                    documentTextRange.Load(fs, System.Windows.DataFormats.Html);

can any body help on this..
Thanks..