using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        // define the collection to hold all data

        List<CityPopulationModel> data = new List<CityPopulationModel>();

        Console.WriteLine("Population per City\n===================");

        // populate data into the collection

        data.Add(new CityPopulationModel { Name = "Manila", Region = "NCR", Population = 10000 });

        data.Add(new CityPopulationModel { Name = "Muntinlupa", Region = "NCR", Population = 20000 });

        data.Add(new CityPopulationModel { Name = "Iloilo", Region = "Region6", Population = 30000 });

        data.Add(new CityPopulationModel { Name = "Bacolod", Region = "Region6", Population = 40000 });

        data.Add(new CityPopulationModel { Name = "Cebu", Region = "Region7", Population = 50000 });

        data.ForEach((x) => Console.WriteLine("Name: {0,-15} Region: {1,-10} Pop: {2,10}", x.Name, x.Region, x.Population));

        Console.WriteLine("\nPopulation per Region\n=====================");

        var dataPerRegion = data

            .GroupBy(x=>x.Region) // group all items in the collection according to the Region

            .Select(g=>new { Region = g.Key, Population = g.Sum(x=>x.Population)}) // each group is given a key (Region in this case), then you can aggregately call operations to its other members/properties (i.e. Sum all Population per group)

            .ToList(); // make a new list given your manipulated data

        dataPerRegion.ForEach((x) => Console.WriteLine("Region: {0,-10} Pop: {1,10}", x.Region, x.Population));

    }

}

​

public class CityPopulationModel

{

    public string Name {get;set;}

    public string Region {get;set;}

    public int Population {get;set;}

}