using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Customer> customers = new List<Customer>() { new Customer() { Name = "Jan", Surname = "Tichy", OrderedProduct = new Product[] { new Product() { ProductCategory = Product.Category.Food, ProductName = "Neco k jidlu" }, new Product() { ProductCategory = Product.Category.Pets, ProductName = "Neco pro psa" }, new Product() { ProductCategory = Product.Category.Pets, ProductName = "Neco pro rybicky" } } }, new Customer() { Name = "Zuzana", Surname = "Mala", OrderedProduct = new Product[] { new Product() { ProductCategory = Product.Category.Pets, ProductName = "Neco pro kocku" } } } };

        var groupProductQuery = from Customer c in customers

                                         from Product p in c.OrderedProduct

                                         group p by p.ProductCategory into ProductGroup

                                         select new {

                                             ProductID = ProductGroup.Key,

                                             CustomerList = (from Customer c in customers

                                                             from Product p in c.OrderedProduct

                                                             where p.ProductCategory == ProductGroup.Key

                                                             select new { Surname = c.Surname }).ToList()

                                         };

​

         Console.WriteLine("Query 5 >>>");

         foreach (var gpq in groupProductQuery) {

             Console.WriteLine("Category: {0}", gpq.ProductID);

             foreach (var cust in gpq.CustomerList) {

                 Console.WriteLine("  {0}", cust.Surname);

             }

         }

        Console.WriteLine("\n---\n");

​

        foreach (var category in Enum.GetNames(typeof(Product.Category)))

        {

            Console.WriteLine("Category: " + category);

            foreach (var cust in customers.Where(c => c.OrderedProduct.Select(p => p.ProductCategory.ToString()).Contains(category)))

                Console.WriteLine("\t" + cust.Surname);

        }

​

        Console.WriteLine("\n---\n");

​

        foreach (var category in Enum.GetValues(typeof(Product.Category)).Cast<Product.Category>())

        {

            Console.WriteLine("Category: " + category.ToString());

            foreach (var cust in customers.Where(c => c.OrderedProduct.Select(p => p.ProductCategory).Contains(category)))

                Console.WriteLine("\t" + cust.Surname);

        }

        Console.WriteLine("\n---\n");

        var categories = Enum.GetNames(typeof(Product.Category)).Select(n => new { Name = n, Customers = customers.Where(c => c.OrderedProduct.Select(p => p.ProductCategory.ToString()).Contains(n)) });

        //var categories = Enum.GetValues(typeof(Product.Category)).Cast<Product.Category>().Select(v => new { Name = v.ToString(), Customers = customers.Where(c => c.OrderedProduct.Select(p => p.ProductCategory).Contains(v)) });

        foreach (var category in categories)

        {

            Console.WriteLine("Category: " + category.Name);

            foreach (var cust in category.Customers)

                Console.WriteLine("\t" + cust.Surname);

        }

        Console.WriteLine("\n---\n");

        Console.WriteLine(String.Join("\n", Enum.GetValues(typeof(Product.Category)).Cast<Product.Category>().Select(pc => "Category: " + pc.ToString() + "\n" + String.Join("\n", customers.Where(c => c.OrderedProduct.Select(p => p.ProductCategory).Contains(pc)).Select(c => "\t" + c.Surname)))));

    }

}

​

public class Product

{

    public enum Category

    {

        Food = 0,

        Pets = 1,

        Toys = 2

    }

    public string ProductName { get; set; }

    public Category ProductCategory { get; set; }

}

public class Customer

{

    public string Name { get; set; }

    public string Surname { get; set; }

    public Product[] OrderedProduct { get; set; }

}

​