I am trying to read a CSV file, and after some calculations, write a line to that same file. But it doesn’t work because it can’t write to a file that is already open. Would Filestream fix this issue? What’s the best way to read and write to the same file?

Yeah, use filestream and ser the access to readwrite
OP’s probably also going to have to manage position seeking, since they want to write to the end (I assume?*) and I don’t think there’s any way to do that while preserving the read position.
* and if not, it’s going to be even more work because you can’t just “replace a line in a file” if the line length is different
Read to consume lazily e.g. iterating over it while consuming (large file):
IEnumerable<string> lines = File.ReadLines(path);
Read into List<string> (small file):
var content = File.ReadAllLines(path);
To store:
“`File.WriteAllLines(path, lines);““
There is 2 overloads, taking either a List<string> or an IEnumerable<string>.
If that doesn’t fit your need, e.g. you need more control and your file is large, use a FileStream._3bX7W3J0lU78fp7cayvNxx{max-width:208px;text-align:center} ._1PeZajQI0Wm8P3B45yshR{fill:var(–newCommunityTheme-actionIcon)}._1PeZajQI0Wm8P3B45yshR._3axV0unm-cpsxoKWYwKh2x{fill:#ea0027} ._1x9diBHPBP-hL1JiwUwJ5J{font-size:14px;font-weight:500;line-height:18px;color:#ff585b;padding-left:3px;padding-right:24px}._2B0OHMLKb9TXNdd9g5Ere-,._1xKxnscCn2PjBiXhorZef4{height:16px;padding-right:4px;vertical-align:top}._1LLqoNXrOsaIkMtOuTBmO5{height:20px;padding-right:8px;vertical-align:bottom}.QB2Yrr8uihZVRhvwrKuMS{height:18px;padding-right:8px;vertical-align:top}._3w_KK8BUvCMkCPWZVsZQn0{font-size:14px;font-weight:500;line-height:18px;color:var(–newCommunityTheme-actionIcon)}._3w_KK8BUvCMkCPWZVsZQn0 ._1LLqoNXrOsaIkMtOuTBmO5,._3w_KK8BUvCMkCPWZVsZQn0 ._2B0OHMLKb9TXNdd9g5Ere-,._3w_KK8BUvCMkCPWZVsZQn0 ._1xKxnscCn2PjBiXhorZef4,._3w_KK8BUvCMkCPWZVsZQn0 .QB2Yrr8uihZVRhvwrKuMS{fill:var(–newCommunityTheme-actionIcon)} ._33axOHPa8DzNnTmwzen-wO{font-size:14px;font-weight:700;letter-spacing:.5px;line-height:32px;text-transform:uppercase;display:block;padding:0 16px;width:100%} .FIYolDqalszTnjjNfThfT{max-width:256px;white-space:normal;text-align:center} ._9ZuQyDXhFth1qKJF4KNm8{padding:12px 12px 40px}._2iNJX36LR2tMHx_unzEkVM,._1JmnMJclrTwTPpAip5U_Hm{font-size:16px;font-weight:500;line-height:20px;color:var(–newCommunityTheme-bodyText);margin-bottom:40px;padding-top:4px}._306gA2lxjCHX44ssikUp3O{margin-bottom:32px}._1Omf6afKRpv3RKNCWjIyJ4{font-size:18px;font-weight:500;line-height:22px;border-bottom:2px solid var(–newCommunityTheme-line);color:var(–newCommunityTheme-bodyText);margin-bottom:8px;padding-bottom:8px}._2Ss7VGMX-UPKt9NhFRtgTz{margin-bottom:24px}._3vWu4F9B4X4Yc-Gm86-FMP{border-bottom:1px solid var(–newCommunityTheme-line);margin-bottom:8px;padding-bottom:2px}._3vWu4F9B4X4Yc-Gm86-FMP:last-of-type{border-bottom-width:0}._2qAEe8HGjtHsuKsHqNCa9u{font-size:14px;font-weight:500;line-height:18px;color:var(–newCommunityTheme-bodyText);padding-bottom:8px;padding-top:8px}.c5RWd-O3CYE-XSLdTyjtI{padding:8px 0}._3whORKuQps-WQpSceAyHuF{font-size:12px;font-weight:400;line-height:16px;color:var(–newCommunityTheme-actionIcon);margin-bottom:8px}._1Qk-ka6_CJz1fU3OUfeznu{margin-bottom:8px}._3ds8Wk2l32hr3hLddQshhG{font-weight:500}._1h0r6vtgOzgWtu-GNBO6Yb,._3ds8Wk2l32hr3hLddQshhG{font-size:12px;line-height:16px;color:var(–newCommunityTheme-actionIcon)}._1h0r6vtgOzgWtu-GNBO6Yb{font-

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Note, that closing the inner stream may close the outer stream when applying the using keyword.
It might help to follow the single-responsibility principle here. How about read from the file using one function that returns the result of that condition, and then a second function that accepts the result(boolean) to decide if the file needs to be written to? This will allow for the file to be closed after reading from it and opened when it is time to write to it.
This sound like the easiest solution.
You have said that should write to the same file.
But is that really the case? I would suggest to pipe the data to another file, and add the new line if needed. And then replace the source file with the new one. This way you can use less memory in the process.
I don’t know compatibility with what you are writing (or OS) but there is an interop library for writing excel files (including csv if you so choose) but I believe you need excel installed. There are also nuget packages like gembox for Net framework so you don’t have to re-invent the wheel.
It looks WriteBBCSV() is in the same using statement as StreamReader. Move WriteBBCSV() outside the using statement. Make sure the reader is closed before trying to write to it.
That’s what I was thinking, but calling that function depends on what’s in the stream read.
C# devs
null reference exceptions

source