using System.Collections.Generic;

using System.Linq;

using System;

public class Program

{

    public  void Main()

    {

            List<string[]> lst = new List<string[]> { new[] { "1", "4", "6", "8" }, new[] { "1", "2", "6", "8" }, new[] { "2", "4", "6", "8" }, new[] { "3", "4", "5", "7" } };

​

            var newList = FilterData(lst);

        Console.WriteLine("---INPUT---\n\n");

        lst.ForEach(li=>{

            Console.WriteLine(string.Join(",", li));

        }); 

        Console.WriteLine("\n\n---OUTPUT---\n");

        newList.ForEach(li=>{

            Console.WriteLine(string.Join(",", li));

        }); 

    }

​

​

  private List<string[]> FilterData(List<string[]> datatable)

        {

            // List is made of String Array, so need string[] variable not list

            string[] previousRow = null ;

            string[] currentRow;

            string[] rowDifferences ;

​

            // zto store the result

            List<string[]> resultingDataset = new List<string[]>();

​

            foreach (var item in datatable)

            {

                if (previousRow == null)

                {

                    previousRow = item;

                    resultingDataset.Add(previousRow); // add first item to list

                    continue;

                }

​

                currentRow = item; 

                // check and replace with "-" if elment exist in previous 

                rowDifferences = currentRow.Select((x, i) => currentRow[i] == previousRow[i] ? "-" : currentRow[i]).ToArray();  

                resultingDataset.Add(rowDifferences);

​

                // make current as previos

                previousRow = item;

            }

            return resultingDataset;

        }

}