/* Description in the comments to the code */ using System; using System.Collections.Generic; using System.Linq; using static System.Console; namespace SoloLearn { //Entities class Book { public int BookId { get; set; } public string Title { get; set; } public List Authors { get; set; } = new List(); } class Author { public int AuthorId { get; set; } public string Name { get; set; } public List Books { get; set; } = new List(); } //Simple Database context /* Implementing a many-to-many * relationship between tables: * Books(BookSet) and * Authors(AuthorSet) */ class Database { public List BookSet { get; set; } = new List(); public List AuthorSet { get; set; } = new List(); //Enable initialization public void SetInitialization() { new Initializing(this); } } //Database context initializing class Initializing { Database db; public Initializing(Database db) { this.db = db; Seed(); } //initial values for database tables void Seed() { List books = new List { new Book { BookId = 1, Title = "Learning C" }, new Book { BookId = 2, Title = "Learning C++" }, new Book { BookId = 3, Title = "Learning C#" }, new Book { BookId = 4, Title = "Learning HTML5 / CSS3" }, new Book { BookId = 5, Title = "Learning JavaScript" }, new Book { BookId = 6, Title = "Learning PHP" }, new Book { BookId = 7, Title = "Learning Java" } }; List authors = new List { new Author { AuthorId = 1, Name = "Adam Tree" }, new Author { AuthorId = 2, Name = "Bob Skywalker" }, new Author { AuthorId = 3, Name = "Clod Sun" }, new Author { AuthorId = 4, Name = "David Water" } }; books[0].Authors.Add(authors[0]); books[0].Authors.Add(authors[1]); books[0].Authors.Add(authors[2]); books[1].Authors.Add(authors[1]); books[2].Authors.Add(authors[2]); books[2].Authors.Add(authors[1]); books[3].Authors.Add(authors[3]); books[4].Authors.Add(authors[0]); books[5].Authors.Add(authors[1]); books[6].Authors.Add(authors[2]); books[6].Authors.Add(authors[1]); authors[0].Books.Add(books[0]); authors[0].Books.Add(books[4]); authors[1].Books.Add(books[0]); authors[1].Books.Add(books[1]); authors[1].Books.Add(books[2]); authors[1].Books.Add(books[5]); authors[1].Books.Add(books[6]); authors[2].Books.Add(books[0]); authors[2].Books.Add(books[2]); authors[2].Books.Add(books[6]); authors[3].Books.Add(books[3]); books.ForEach(b => db.BookSet.Add(b)); authors.ForEach(a => db.AuthorSet.Add(a)); } } /////////// TESTING ////////////// class Program { static void Main(string[] args) { Database database = new Database(); database.SetInitialization(); //*** READ ***////////// //Get all books //////// WriteLine("GET ALL BOOKS"); var books = database.BookSet; foreach (var book in books) { Write($"\n\"{book.Title}\"\n by "); foreach (var author in book.Authors) { Write($"{author.Name}{(author == book.Authors.Last() ? "" : ", ")}"); } WriteLine(); } //Get the books of a certain author ////////////// WriteLine("\nGet all the books by Bob Skywalker".ToUpper()); //First way var bobBooks = database.BookSet .Where( b => b.Authors.Any(a => a.Name == "Bob Skywalker") ); foreach (var book in bobBooks) { WriteLine($"{book.Title}"); } WriteLine("\nGet all the books by Adam Tree".ToUpper()); //Second way var adamBooks = database.AuthorSet .FirstOrDefault(a => a.Name == "Adam Tree")?.Books; foreach (var book in adamBooks) { WriteLine($"{book.Title}"); } ///Get all authors and sort the result /// WriteLine("\nGet all authors and sort the result".ToUpper()); var authors = database.AuthorSet .OrderBy(a => a.Name); foreach (var author in authors) { WriteLine($"{author.AuthorId} {author.Name}"); } //*** CREATE ***///// //Add a new author and his book// WriteLine("\nAdd a new author and his book".ToUpper()); Author newAuthor = new Author { AuthorId = 5, Name = "Edgar Rain" }; // If such an author is not in the database, //then write to the database var tmp = database.AuthorSet.FirstOrDefault(a => a.Name == newAuthor.Name); if (tmp == null) database.AuthorSet.Add(newAuthor); //Add a book Book newAuthorsBook = new Book { BookId = 8, Title = "Learning Python", Authors = database.AuthorSet.Where(a => a.Name == "Edgar Rain").ToList() }; database.BookSet.Add(newAuthorsBook); database.AuthorSet.FirstOrDefault(a => a.Name == "Edgar Rain")?.Books.Add(newAuthorsBook); //Updated book list WriteLine("Updated book list"); var books2 = database.BookSet; foreach (var book in books2) { Write($"\n\"{book.Title}\"\n by "); foreach (var author in book.Authors) { Write($"{author.Name}{(author == book.Authors.Last() ? "" : ", ")}"); } WriteLine(); } //******UPDATE ****/// //Change the name of Adam Tree to Adam Tree Jr. var adamTree = database.AuthorSet.FirstOrDefault(a => a.Name == "Adam Tree"); if (adamTree != null) adamTree.Name = "Adam Tree Jr."; WriteLine("\nChange the name of Adam Tree to Adam Tree Jr.".ToUpper()); //Updated book list WriteLine("Updated book list"); foreach (var book in database.BookSet) { Write($"\n\"{book.Title}\"\n by "); foreach (var author in book.Authors) { Write($"{author.Name}{(author == book.Authors.Last() ? "" : ", ")}"); } WriteLine(); } ///// To be continued... } } }

Database Context Imitation