Converts an arbitrary Excel spreadsheet to a CSV file using MS Office Interop

Excel2CSV.cs

using System;
using System.IO;
using System.Linq;
using Microsoft.Office.Interop.Excel;

namespace Excel2CSV
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 2)
            {
                Console.WriteLine("Invalid number of arguments");
                Console.WriteLine("Usage: excel2csv.exe source destination [columns]");
                Console.WriteLine("Example: excel2csv.exe example.xls output.csv A B F");
                return;
            }

            var source = args[0].Contains(":") ? args[0] : Path.Combine(Directory.GetCurrentDirectory(), args[0]);
            var destination = args[1].Contains(":") ? args[1] : Path.Combine(Directory.GetCurrentDirectory(), args[1]);
            var columnsToKeep = args.Skip(2).ToList();

            var app = new Application { DisplayAlerts = false };
            var workbook = app.Workbooks.Open(source);

            if (columnsToKeep.Any())
                workbook.RemoveAllColumnsExcept(columnsToKeep);

            workbook.SaveAs(destination, XlFileFormat.xlCSVWindows);
            workbook.Close(false);
        }
    }
}

WorkbookExtensionMethods.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Office.Interop.Excel;

namespace Excel2CSV
{
    internal static class WorkbookExtensionMethods
    {
        public static void RemoveAllColumnsExcept(this _Workbook workbook, IEnumerable<string> columnsToKeep)
        {
            var columnCount = (int)workbook.ActiveSheet.UsedRange.Columns.Count;
            var allColumns = Enumerable.Range(1, columnCount).Select(ConvertToLetters);
            var columnsToRemove = allColumns.Except(columnsToKeep);
            workbook.RemoveColumns(columnsToRemove.ToList());
        }

        public static void RemoveColumns(this _Workbook workbook, IEnumerable<string> columns)
        {
            var selection = string.Join(";", columns.Select(c => c + "1"));
            workbook.RemoveColumns(selection);
        }

        public static void RemoveColumns(this _Workbook workbook, string selection)
        {
            var range = workbook.ActiveSheet.Range[selection];
            range.EntireColumn.Delete();
        }

        private static string ConvertToLetters(int index)
        {
            const int alphabetSize = 26;
            const int start = 'A';

            if (index < 1)
                return string.Empty;

            var offset = (index - 1) % alphabetSize;
            return ConvertToLetters(index - alphabetSize - offset) + (char)(start + offset);
        }
    }
}