Apache POI provides Java APIs for manipulating file formats based on Open Office XML standards and Microsoft’s OLE 2 Compound Document format (.xls/.xlsx). Basically it lets you read and write Excel documents in Java. NPOI is a .NET library that lets you read/write Office formats without COM or interop, and dotnetcore/NPOI is a port of NPOI to dotnet core.
If you only care about Excel 2007+ (.xlsx) then EPPlus is a great option.
CSV Helper is a nuget package that makes it easy to read and write CSV files.