Using EPPlus Library to Convert XLSX to CSV

The EPPlus library for Excel data is great, but it has no built-in functionality for converting .xlsx to .csv.

Here is a sample (posted here because I discovered a pitfall, and it's not a slam dunk) for doing just that.  It's a single file, console app, just add EPPlus via nuget.

Update: Prior code was no good for large files (and said so).  Updated to work with giant files.