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NPOI leaks memory big time #262
Description
Given a pretty large Excel file (160MB, with millions and millions of rows).
You can't even open it because it's going to spam the heap with hundreds of millions of objects.
I made a little test, and tried to pass instantiation but I always ran out of memory:
Activity
mganss commentedon Jun 30, 2023
Can you open it using NPOI alone somehow?
tonyqus commentedon Sep 28, 2023
160M Excel file is too huge. If you unzip the Excel (rename the .xlsx to .zip first), you will see how huge the size of openxml files are. Why don't you use CSV instead for this kind of data export.
mercenaryntx commentedon Sep 29, 2023
It's not my call, obviously. The Excel file is given as an external resource, and the number one rule of working with files is not to load them into memory. That's why we have streams in .NET in the first place. I switched to ExcelDataReader and all my problems disappeared. No offense, but building this lib on NPOI was clearly a mistake.
tonyqus commentedon Sep 29, 2023
@mercenaryntx Good to know that ExcelDataReader solves your issue. The NPOI community is working hard on performance improvement.
But there is a tradeoff between supporting more Excel features and performance. Definitely, NPOI can read the file better if we don't take a few properties from Excel into consideration. Usally, this kind of huge file reading is optimized by reading only partial data of openxml (without cell style info and other advanced properties).
Anyway, it's good to hear that you solve your issue. Good luck with ExcelDataReader.
akonyer commentedon Oct 1, 2025
We are constantly running into OOM issues with this library when trying to parse medium-large size excel files. Ours are not even that big. currently at 1255 KB on a 3GB server and it crashes it. Something is wrong with this library...
mganss commentedon Oct 2, 2025
@akonyer Does this occur with NPOI alone, too?