LINQ layer
Use LINQ for data filtering and reporting.
Finding books with LINQ
Adding a FindByAuthor method to Library lets you search by author using LINQ Where. The signature: public IEnumerable<Book> FindByAuthor(string author).
In the body write _books.Where(b => b.Author == author). It returns matching books as a lazy sequence, produced only when the caller starts a foreach.
Lazy sequences and foreach
The result of Where is an IEnumerable<Book>; it is not computed at once. A foreach yields the next match each step. On large lists this saves memory.
The result can be empty; foreach may not loop at all. You can also take a single result with FirstOrDefault(), which returns null when empty.
Exercise Add the FindByAuthor method
Add a FindByAuthor method to the Library class below: it filters by author using Where and returns IEnumerable<Book>. In Main add two books, call FindByAuthor and print titles with foreach.
Show example solution
Write public IEnumerable<Book> FindByAuthor(string author) { return _books.Where(b => b.Author == author); }. In Main write foreach (var b in lib.FindByAuthor("Dostoevsky")) Console.WriteLine(b.Title);
Example solution:
using System;
using System.Collections.Generic;
using System.Linq;
class Book
{
public string Title { get; set; }
public string Author { get; set; }
}
class Library
{
private List<Book> _books = new List<Book>();
public void Add(Book b) => _books.Add(b);
public int Count => _books.Count;
public IEnumerable<Book> FindByAuthor(string author)
{
return _books.Where(b => b.Author == author);
}
}
class Program
{
static void Main()
{
var lib = new Library();
lib.Add(new Book { Title = "Crime and Punishment", Author = "Dostoevsky" });
lib.Add(new Book { Title = "Karamazov Brothers", Author = "Dostoevsky" });
foreach (var b in lib.FindByAuthor("Dostoevsky"))
{
Console.WriteLine(b.Title);
}
}
}Test yourself
-
Match the LINQ operator to its behavior
- Where
- Filter by condition
- Select
- Shape transform
- OrderBy
- Sort
- Count
- Return tally
-
Complete the Where call
return _books.____(b => b.Author == author);Show answer
return _books.Where(b => b.Author == author); -
What does Where return?
- A single value
- A sorted sequence
- Items matching the condition
- Groups
Show answer
Items matching the condition — Correct. Where returns items matching the condition as a lazy sequence.