using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

            string inputcsv = @"Id,Field1,Field2,Country,Field3

1,one,two,Australia,three

2,one,two,New Zealand,three

3,one,two,Indonesia,three

4,one,two,China,three

5,one,two,Japan,three

";

​

            string masterCsv = @"Field1,Country,Field2

one,Indonesia,...

one,China,...

one,Japan,...

";

​

            string errorCsv = "";

​

            // For all in inputCsv where the country value is not listed in the masterCsv

            // Write to errorCsv

​

            bool csvHasHeader = true;

            int countryIndexInMaster = 1;

            char delimiter = ',';

            List<string> countries = new List<string>();

            using (var masterReader = new System.IO.StringReader(masterCsv))

            {

                string line = null;

                if (csvHasHeader)

                {

                    line = masterReader.ReadLine();

                    if(line != null)

                        countryIndexInMaster = line.Split(delimiter).ToList().FindIndex(x => x.Trim('"').Equals("Country", StringComparison.OrdinalIgnoreCase));

                }

                while ((line = masterReader.ReadLine()) != null)

                {

                    string country = line.Split(delimiter)[countryIndexInMaster].Trim('"');

                    if (!countries.Contains(country))

                        countries.Add(country);

                }

            }

​

            // Read the input CSV, if the country is not in the master list "countries", write it to the errorCsv

            int countryIndexInInput = 3;

            csvHasHeader = true;

​

            var outputStringBuilder = new System.Text.StringBuilder();

            using (var outputWriter = new System.IO.StringWriter(outputStringBuilder))

            using (var inputReader = new System.IO.StringReader(inputcsv))

            {

                string line = null;

                if (csvHasHeader)

                {

                    line = inputReader.ReadLine();

                    if (line != null)

                    {

                        countryIndexInInput = line.Split(delimiter).ToList().FindIndex(x => x.Trim('"').Equals("Country", StringComparison.OrdinalIgnoreCase));

                        outputWriter.WriteLine(line);

                    }

                }

                while ((line = inputReader.ReadLine()) != null)

                {

                    string country = line.Split(delimiter)[countryIndexInInput].Trim('"');

                    if(!countries.Contains(country))

                    {

                        outputWriter.WriteLine(line);

                    }

                }

                outputWriter.Flush();

                errorCsv = outputWriter.ToString();

            }

​

            // dump output to the console

            Console.WriteLine(errorCsv);

​

    }

}