using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        //db.Articles

        List<Article> articles = new List<Article> {

            new Article { ArticleId = 1, ArticleName = "Article1" },

            new Article { ArticleId = 2, ArticleName = "Article2" },

            new Article { ArticleId = 3, ArticleName = "Article3" },

        };

        //db.Categories

        List<Category> categories = new List<Category> {

            new Category { CategoryId = 1, CategoryName = "Category1" },

            new Category { CategoryId = 2, CategoryName = "Category2" },

            new Category { CategoryId = 3, CategoryName = "Category3" },            

        };

        //db.ArticleCategories

        List<ArticleCategory> articleCategories = new List<ArticleCategory> {

            new ArticleCategory { ArticleId = 1, CategoryId = 1 },

            new ArticleCategory { ArticleId = 2, CategoryId = 2 },

            new ArticleCategory { ArticleId = 2, CategoryId = 3 },

            new ArticleCategory { ArticleId = 3, CategoryId = 2 },

        };

        //var articles = db.Articles;

        //var categories = db.Categories;

        //var articleCategories = db.ArticleCategories;

        var query = from ac in articleCategories

            join a in articles on ac.ArticleId equals a.ArticleId

            join c in categories on ac.CategoryId equals c.CategoryId

            group c by a.ArticleName into g

            select new { ArticleName = g.Key, Categories = g.ToList() };

        foreach(var a in query)

        {

            Console.WriteLine(a.ArticleName);

            foreach(var c in a.Categories)

                Console.WriteLine("  " + c.CategoryName);

        }       

    }

}

​

public class Article

{

    public int ArticleId { get; set; }

    public string ArticleName { get; set; }

}

​

public class ArticleCategory

{

    public int ArticleId { get; set; }

    public int CategoryId { get; set; }

}

​

public class Category

{

    public int CategoryId { get; set; }

    public string CategoryName { get; set; }

}