csv parser

CSV Helper is a nuget package that makes it easy to read and write CSV files.

excel parser

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.