using System;

using System.Linq;

using System.Collections.Generic;

​

    public class DataItem

{

    public object Response { get; set; }

    public string ErrorInfo { get; set; }

    public string Faretype { get; set; }

    public string ServiceProvider { get; set; }

}               

public class Program

{

    public static void Main()

    {

        List<DataItem> data = new List<DataItem>

        {

            new DataItem { Response = "fares1", ErrorInfo = null, Faretype = "STO", ServiceProvider = "AMADS" },

            new DataItem { Response = null, ErrorInfo = "not null", Faretype = "STO", ServiceProvider = "AMADS" },

            new DataItem { Response = null, ErrorInfo = "not null", Faretype = "ITR", ServiceProvider = "AMADS" },

            new DataItem { Response = "fares2", ErrorInfo = null, Faretype = "ITR", ServiceProvider = "AMADS" },

            new DataItem { Response = null, ErrorInfo = "not null", Faretype = "ITR", ServiceProvider = "AMADS" },

            new DataItem { Response = null, ErrorInfo = "not null", Faretype = "PB", ServiceProvider = "AMADS" },

            new DataItem { Response = null, ErrorInfo = "not null", Faretype = "PB", ServiceProvider = "AMADS" },

            new DataItem { Response = null, ErrorInfo = "not null", Faretype = "PB", ServiceProvider = "NDCABA" },

            new DataItem { Response = null, ErrorInfo = "not null", Faretype = "PB", ServiceProvider = "NDCABA" },

        };

​

         var result = data

            .GroupBy(x => new { x.ServiceProvider, x.Faretype })

            .SelectMany(group => group

                .Where(item => item.Response != null || group.All(x => x.Response == null)))

            .Select(item => new { item.Response, item.Faretype, item.ServiceProvider })

            .Distinct();

​

        foreach (var item in result)

        {

            Console.WriteLine("response: "+item.Response+", Faretype: "+item.Faretype+", ServiceProvider: "+item.ServiceProvider);

        }

    }

}

​

​