I strongly believe that
Uroš Šmon[
^] is right as to this statement: "
Csv is very simple text format, that knows nothing about its content". You did not provide a way you save numbers into csv file, so we can't help you to resolve your issue, because we have no access to your screen or data. I tested below code and it works perfect, even if Excel displays the numbers in scientic format.
decimal[] numbers = {43564765876978, 56576867897699, 45765879789789};
string path = @"D:\text.csv";
string[] lines = numbers.Select(x=>x.ToString()).ToArray();
File.WriteAllLines(path, lines);
lines = File.ReadAllLines(path);