Inconsistent formatting when reading multiple XLS files concurrently

[Vemahk]

NPOI Version

2.7.2

File Type

• XLSX
• XLS
• DOCX
• XLSM
• OTHER

Reproduce Steps

Reproducible code + spreadsheet:
Demo.zip

Excel File only:
test.xls

Create a XLS spreadsheet with a date+time column. Copy the date time column one over. Format the first column to show only dates, and format the second to show only times. Then, in two different threads, open the spreadsheet and parse with NPOI.

This is done in the above Demo.zip

Issue Description

When reading the same XLS file (or two similar ones) on different threads, cell values are inconsistently formatted when read using DataFormatter.FormatCellValue(ICell). I believe this bug was introduced with 96186d6. This was fixed in upstream POI with apache/poi@00c69b3.

[Bykiev]

Hi, nice catch, would you like to contribute?

[Vemahk]

Yeah I can take a crack at it. Made the initial bug report as part of my day job, but I got some free time and remember my way around. PR heading y'all's way in a little bit.

[Vemahk]

I'm out of time for tonight, but I at least got a test case, benchmark, and the "simple solution" done. I want to still go back and fix, or at least reduce, the performance regression when I have some more time. Or someone else can pick it up from there if I never get the chance.

[Bykiev]

I thought you'll port the changes with a ThreadLocal from apache/poi@00c69b3

[Vemahk]

If I get some time today, that's my goal. I've never used ThreadLocal or AsyncLocal in C#, though, so I wanted to spend more time figuring it out and making sure it worked.