CSV File Scanner POC. The goal is to minimize time and memory by using an ArrayPool<byte> and File.ReadLines.

Program.cs

using System;
using System.Buffers;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            const string csvFilename = @".\Data\noniouzipcodes2015.csv";

            var scanner = new CsvFileScanner(csvFilename, 9);

            scanner.Execute();
        }

        
    }

    public class CsvFileScanner
    {
        private readonly FileInfo _csvFileInfo;
        private readonly int _numberOfCommas;
        private readonly ArrayPool<byte> _arrayPool;

        public CsvFileScanner(string csvFilename, int numberOfCommas)
        {
            _numberOfCommas = numberOfCommas;
            _arrayPool = ArrayPool<byte>.Shared;
            _csvFileInfo = new FileInfo(csvFilename);
        }

        private byte[] FindCommasInLine(string line, byte[] nums)
        {
            byte counter = 0;

            for (byte index = 0; index < line.Length; index++)
            {
                if (line[index] == ',')
                {
                    nums[counter++] = index;
                }
            }

            return nums;
        }

        public void Execute()
        {
            var hashSet = new HashSet<string>();

            try
            {
                if (!_csvFileInfo.Exists) return;
                
                foreach (var line in File.ReadLines(_csvFileInfo.FullName, Encoding.UTF8))
                {
                    var tempBuffer = _arrayPool.Rent(_numberOfCommas);
                    
                    try
                    {
                        var commaIndexes = FindCommasInLine(line, tempBuffer);
                        hashSet.Add(line.Substring(commaIndexes[1] + 1, commaIndexes[2]));
                    }
                    finally
                    {
                        _arrayPool.Return(tempBuffer, true);

                    }
                }

                foreach (var name in hashSet)
                {
                    Console.WriteLine(name);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine("The process failed: {0}", e.ToString());
            }
        }
    }
}

The idea for this came from Strings Are Evil, where Indy Singh introduced me to the ArrayPool<T>