using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Categories> categories = new List<Categories>

            {

                new Categories { Id =1, CateId =2, CateName = "Cat-B" },

                new Categories { Id =2, CateId =1, CateName = "Cat-A" },

                new Categories { Id =3, CateId =1, CateName = "Cat-A" },

            };

​

            List<Products> products = new List<Products>

            {

                new Products { CategoryId = 1, ProductName ="Product B" },

                new Products { CategoryId = 2, ProductName ="Product B" },

                new Products { CategoryId = 2, ProductName ="Product C" }

            };

​

            var query = categories.GroupBy(x => new { x.Id, x.CateId, x.CateName })

                                   .Select(x => new

                                       {

                                           x.Key.Id,

                                           x.Key.CateId,

                                           x.Key.CateName,

                                           ProductsNames = products.Where(p => p.CategoryId == x.Key.CateId)

                                                                  .Select(p => p.ProductName).ToList()

                                       }).ToList();

        foreach (var item in query)

            {

                Console.WriteLine("Id: {0}, CateId: {1}, CateName: {2}", item.Id, item.CateId, item.CateName);

                foreach (var productName in item.ProductsNames)

                {

                    Console.WriteLine(productName);

                }

            }

    }

}

​

 public class Categories

    {

        public int Id { get; set; }

        public int CateId  { get; set; }

        public string CateName { get; set; }

    }

​

    public class Products

    {

        public int CategoryId { get; set; }

        public string ProductName { get; set; }

    }