Creating Excel spreadsheets .XLS and .XLSX in C# - ASP.NET MVC Project

Recently I had to implement some code to create an Excel spreadsheet/report using C#. The task was: given an Excel spreadsheet template - a .XLS file (with formulas, pivot tables, macros, etc) I had to fill some data in one of the sheets of the spreadsheet and send this modified spreadsheet back to the user requesting such an operation (Excel report). I had a great time while studying the possible ways of doing what the task asks for. In this article I present an ASP.NET MVC project that shows you how to create an .XLS file using NPOI and an .XLSX file using ExcelPackage.