using System;

using System.Data;

using System.Linq;

public class Program

{

    public static void Main()

    {

        DataTable dt = new DataTable();

        dt.Columns.Add("Id", typeof(int));

        dt.Columns.Add("Text", typeof(string));

        dt.Columns.Add("Value1", typeof(string));

        dt.Columns.Add("Value2", typeof(int));

​

        dt.Rows.Add(1, "Test1", "584", 12);

        dt.Rows.Add(2, "Test2", "32", 123);

        dt.Rows.Add(3, "Test3", "425", 54);

        dt.Rows.Add(4, "Test1", "4", 755);

        dt.Rows.Add(5, "Test5", "854", 879);

        dt.Rows.Add(6, "Test2", "1", null);

        dt.Rows.Add(7, "Test2", "999", 3);

​

        var duplicates = dt.Rows.OfType<DataRow>().GroupBy(x => x[1]).Where(x => x.Count() > 1).ToList();

​

        //get the current highestId (first column) so that when we remove duplicates and a new row the new row will get the next available id

        var highestId = dt.Rows.OfType<DataRow>().Max(x => (int)x[0]);

​

        //enumerate all duplicates

        foreach (var duplicate in duplicates)

        {

            //get the highest value of each column

            var newId = ++highestId;

            var newText = duplicate.Key;

            var newValue1 = duplicate.Max(x => x[2]); //this does a string comparison, instead of a numeric one, this means that for example that 2 is bigger then 10

​

            // use this if you need numeric comparison

            var newValue1AsNumeric = duplicate.Select(x =>

            {

                if (int.TryParse(Convert.ToString(x[2]), out var value))

                    return value;

​

                return (int?)null;

            }).OfType<int>().Max(); 

​

            var newValue2 = duplicate.Select(x => x[3]).OfType<int>().Max();

​

            //enumerate each row of the duplicate

            foreach (var dataRow in duplicate)

                dt.Rows.Remove(dataRow);

​

            dt.Rows.Add(newId, newText, newValue1, newValue2);

        }

        foreach (DataRow dr in dt.Rows)

        {

            foreach (var item in dr.ItemArray)

            {

                Console.Write(item);

                Console.Write("|");

            }

            Console.WriteLine();

        }

    }

}