[c#] How to read file using NPOI

As Janoulle pointed out, you don't need to detect which extension it is if you use the WorkbookFactory, it will do it for you. I recently had to implement a solution using NPOI to read Excel files and import email addresses into a sql database. My main problem was that I was probably going to receive about 12 different Excel layouts from different customers so I needed something that could be changed quickly without much code. I ended up using Npoi.Mapper which is an awesome tool! Highly recommended!

Here is my complete solution:

using System.IO;
using System.Linq;
using Npoi.Mapper;
using Npoi.Mapper.Attributes;
using NPOI.SS.UserModel;

namespace JobCustomerImport.Processors
{
    public class ExcelEmailProcessor
    {
        private UserManagementServiceContext DataContext { get; }

        public ExcelEmailProcessor(int customerNumber)
        {
            DataContext = new UserManagementServiceContext();
        }

        public void Execute(string localPath, int sheetIndex)
        {
            IWorkbook workbook;
            using (FileStream file = new FileStream(localPath, FileMode.Open, FileAccess.Read))
            {
                workbook = WorkbookFactory.Create(file);
            }

            var importer = new Mapper(workbook);
            var items = importer.Take<MurphyExcelFormat>(sheetIndex);
            foreach(var item in items)
            {
                var row = item.Value;
                if (string.IsNullOrEmpty(row.EmailAddress))
                    continue;

                UpdateUser(row);
            }

            DataContext.SaveChanges();
        }

        private void UpdateUser(MurphyExcelFormat row)
        {
            //LOGIC HERE TO UPDATE A USER IN DATABASE...
        }

        private class MurphyExcelFormat
        {
            [Column("District")]
            public int District { get; set; }

            [Column("DM")]
            public string FullName { get; set; }

            [Column("Email Address")]
            public string EmailAddress { get; set; }

            [Column(3)]
            public string Username { get; set; }

            public string FirstName
            {
                get
                {
                    return Username.Split('.')[0];
                }
            }

            public string LastName
            {
                get
                {
                    return Username.Split('.')[1];
                }
            }
        }
    }
}

I am so happy with NPOI + Npoi.Mapper (from Donny Tian) as an Excel import solution that I wrote a blog post about it, going in to more detail about this code above. You can read it here if you wish: Easiest way to import excel files. The best thing about this solution is that it runs perfectly in a serverless azure/cloud environment which I couldn't get with other Excel tools/libraries.