MVC Controllers: BooksController.cs (Cont.)
using System;
The namespace contains fundamental classes and base classes that define commonly used value and reference data types, events and event handlers, interfaces, attributes, and processing exceptions.

using System.Collections.Generic;
The namespace contains interfaces and classes that define generic collections, which allow users to create strongly typed collections that provide better type safety and performance than non-generic strongly typed collections.

using System.Linq;
Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language.

using System.Threading.Tasks;
The Task class represents a single operation that does not return a value and that usually executes asynchronously.

using Microsoft.AspNetCore.Mvc;
It provides types required to build an MVC app.

using Microsoft.EntityFrameworkCore;
Entity Framework allows developers to work with relational data using domain-specific objects, eliminating the need for most of the data-access code that developers usually need to write. 

 // GET: Books
 public async Task<IActionResult> Index( ) {
   return View( await _context.Book.ToListAsync( ) );
 }

async Task<IActionResult>
The async keyword marks a method as asynchronous, enabling the use of await within it.

await _context.Book.ToListAsync( )
  • The await keyword pauses the execution of the method until the awaited task completes, without blocking the thread.
  • The ToListAsync method to creates a List<T> by enumerating it asynchronously. 

 // GET: Books/Details/5
 public async Task<IActionResult> Details( int? id ) {
   if ( id == null ) { return NotFound( ); }
   var book = await _context.Book.FirstOrDefaultAsync( m => m.Id == id );
   if ( book == null ) { return NotFound( ); }
   return View( book );
 }

[ ]
The [ ] (square bracket) notation is primarily used for Data Annotations. These are attributes applied to properties or classes in your model to define validation rules, display metadata, or specify how data should be handled.

int? id
It means the id might not always have a value, such as optional foreign keys or when the value is not yet assigned.

m => m.Id == id
It is a lambda expression often used in data access technologies like LINQ (Language Integrated Query) and Entity Framework:

  • Lambda Operator (=>): This separates the input parameters from the expression body.
  • m: This is a parameter representing an object, such as a row in a database table or an item in a list.
  • m.Id: This refers to the “Id” property of the object represented by “m.” This is commonly used for a primary key.
  • ==: This is the equality operator, comparing the value of m.Id with the value of id.
  • id: This is a variable that typically holds the ID you are trying to match.

int? id
It means the id might not always have a value, such as optional foreign keys or when the value is not yet assigned.

using ..;

using ..;

using ..;

using ..;

using ..;

using ..;

C:\ASP.NET-workspace\BookStore1\Controllers\BooksController.cs 
 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.EntityFrameworkCore;
 using BookStore1.Data;
 using BookStore1.Models;

 namespace BookStore1.Controllers {
   public class BooksController : Controller {
     private readonly BookStore1Context _context;
     public BooksController( BookStore1Context context ) { _context = context; }

     // GET: Books
     public async Task<IActionResult> Index( ) {
       return View( await _context.Book.ToListAsync( ) );
     }

     // GET: Books/Details/5
     public async Task<IActionResult> Details( int? id ) {
       if ( id == null ) { return NotFound( ); }
       var book = await _context.Book.FirstOrDefaultAsync( m => m.Id == id );
       if ( book == null ) { return NotFound( ); }
       return View( book );
     }

     // GET: Books/Create
     public IActionResult Create( ) { return View( ); }

     // POST: Books/Create
     // To protect from overposting attacks,
     //   enable the specific properties you want to bind to.
     // For more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
     [ HttpPost ]
     [ ValidateAntiForgeryToken ]
     public async Task<IActionResult> Create(
         [ Bind( "Id, Title, Genre, Price, PublishDate" ) ] Book book ) {
       if ( ModelState.IsValid ) {
         _context.Add( book );
         await _context.SaveChangesAsync( );
         return RedirectToAction( nameof( Index ) );
       }
       return View( book );
     }

     // GET: Books/Edit/5
     public async Task<IActionResult> Edit( int? id ) {
       if ( id == null ) { return NotFound( ); }
       var book = await _context.Book.FindAsync( id );
       if ( book == null ) { return NotFound( ); }
       return View( book );
     }

     // POST: Books/Edit/5
     // To protect from overposting attacks,
     //   enable the specific properties you want to bind to.
     // For more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
     [ HttpPost ]
     [ ValidateAntiForgeryToken ]
     public async Task<IActionResult> Edit( int id,
         [ Bind( "Id, Title, Genre, Price, PublishDate" ) ] Book book ) {
       if ( id != book.Id ) { return NotFound( ); }
       if ( ModelState.IsValid ) {
         try {
           _context.Update( book );
           await _context.SaveChangesAsync( );
         }
         catch ( DbUpdateConcurrencyException ) {
           if ( !BookExists( book.Id ) ) { return NotFound( ); }
           else { throw; }
         }
         return RedirectToAction( nameof( Index ) );
       }
       return View ( book );
     }

     // GET: Books/Delete/5
     public async Task<IActionResult> Delete( int? id ) {
       if ( id == null ) { return NotFound( ); }
       var book = await _context.Book.FirstOrDefaultAsync( m => m.Id == id );
       if ( book == null ) { return NotFound( ); }
       return View( book );
     }

     // POST: Books/Delete/5
     [ HttpPost, ActionName( "Delete" ) ]
     [ ValidateAntiForgeryToken ]
     public async Task<IActionResult> DeleteConfirmed( int id ) {
       var book = await _context.Book.FindAsync( id );
       if ( book != null ) { _context.Book.Remove( book ); }
       await _context.SaveChangesAsync( );
       return RedirectToAction( nameof( Index ) );
     }

     private bool BookExists( int id ) {
       return _context.Book.Any( e => e.Id == id );
     }
   }
 }




      “By the time a man is wise enough to watch his step,    
      he’s too old to go anywhere.”    
      — Billy Crystal on aging