Linq GroupJoin

.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;


namespace tutorials
{

    class Customer
    {
        public string City { get; set; }
        public string Title { get; set; }
    }

    class Provider
    {
        public string City { get; set; }
        public string Title { get; set; }
    }
    class MainClass
    {
        public static void Main(string[] args)
        {


            var customers = new List<Customer>
            {
                new Customer { Title = "EPAM", City = "Krasnodar" },
                new Customer { Title = "SML", City = "Krasnodar" },
                new Customer { Title = "MailRu", City = "Moscow" },
                new Customer{ Title = "Rambler", City = "Moscow"},
            };


            var providers = new List<Provider>
            {
                new Provider{ Title = "Kuban Milk", City = "Krasnodar" },
                new Provider{ Title = "Kuban products", City = "Krasnodar" },
                new Provider{ Title = "Moscow products", City = "Moscow" },
                new Provider{ Title = "Moscow milk", City = "Moscow" },
            };


            var query = customers.GroupJoin(
                providers,                          //  Second Collection for join  
                customer => customer.City,          // Join element from first collection
                provider => provider.City,          // Join element from second collection
                (customer, providerCollection) =>   // Result element
                new {
                    Customer = customer,
                    Providers = providerCollection
                });

            foreach (var item in query)
            {
                Console.WriteLine($"{item.Customer.Title} from {item.Customer.City}");
                foreach(var provider in item.Providers)
                {
                    Console.WriteLine($"    {provider.Title} from {provider.City}");
                }
            }

            /*
             EPAM from Krasnodar
                Kuban Milk from Krasnodar
                Kuban products from Krasnodar
            SML from Krasnodar
                Kuban Milk from Krasnodar
                Kuban products from Krasnodar
            MailRu from Moscow
                Moscow products from Moscow
                Moscow milk from Moscow
            Rambler from Moscow
                Moscow products from Moscow
                Moscow milk from Moscow
             */

        }
    }
}