I'm using version 4.0.0 and get an error while trying to load a CSV file in which one of the cell contents is a period (.). You can reproduce the problem by creating a file with just a . as the single character in the file, then try to load it as a CSV using Workbook.open("c:\temp\file.csv", FileFormatType.CSV). The following error occurs:
An unhandled exception of type 'System.FormatException' occurred in mscorlib.dll
Additional information: Input string was not in a correct format.