Get distinct items by combination from list C#

Question

My object like like this

    public class Region
{
    public Region();

    public string City { get; set; }
    public int PostCode { get; set; }
    public string WfRegion { get; set; }
}

I have a list of this objects in this class Where data is like this

Rodney , 7845 , Auckland
Rodney , 3435 , Auckland
Rodney , 4566 , Auckland
Rodney , 3445 , North Island

I want to filter this list so that I can get an output like this

    Rodney , 7845 , Auckland
    Rodney , 3445 , North Island

(all the possible combination of city and region regardless of postcode). I have wrote some query like this

      var cities = regionsData.DistinctBy(p => 
p.WfRegion).DistinctBy(p=>p.PostCode).DistinctBy(p => p.City).ToList();

But this is giving me a result of first item only like this

        Rodney , 7845 , Auckland

How can I solve this issue?

Answer 1

You need to use GroupBy

var result = regionsData.GroupBy(p => new {p.WfRegion, p.City})
    .Select(g => g.First())
    .ToList();

That will give you groupings by the region and city and then you can just select the first item in each group.

Answer 2

You can use DistinctBy to solve this as follows:

var cities = regionsData.DistinctBy(x => (x.City, x.WfRegion));

Note that this is using C#7 tuple syntax. For older versions you must use an anonymous type as follows:

var cities = regionsData.DistinctBy(x => new {x.City, x.WfRegion});

Full console example:

using System;
using System.Collections.Generic;
using MoreLinq;

namespace ConsoleApp1
{
    public class Region
    {
        public string City     { get; set; }
        public int    PostCode { get; set; }
        public string WfRegion { get; set; }

        public override string ToString()
        {
            return  $"City:{City}, PostCode:{PostCode}, WfRegion:{WfRegion}";
        }
    }

    class Program
    {
        static void Main()
        {
            IEnumerable<Region> regions = new []
            {
                new Region { City = "CityOne", PostCode = 1, WfRegion = "WfRegionOne"},
                new Region { City = "CityOne", PostCode = 2, WfRegion = "WfRegionTwo"},
                new Region { City = "CityTwo", PostCode = 3, WfRegion = "WfRegionOne"},
                new Region { City = "CityOne", PostCode = 4, WfRegion = "WfRegionOne"},
                new Region { City = "CityOne", PostCode = 5, WfRegion = "WfRegionThree"},
                new Region { City = "CityTwo", PostCode = 6, WfRegion = "WfRegionOne"},
                new Region { City = "CityTwo", PostCode = 7, WfRegion = "WfRegionThree"}
            };

            var result = regions.DistinctBy(x => (x.City, x.WfRegion));

            Console.WriteLine(string.Join("\n", result));
        }
    }
}