File I/O

Save and load data as JSON.

Save the Library to a file

A Save(string path) method turns the _books list into text with JsonSerializer.Serialize and writes it to a file with File.WriteAllText. The library then persists across restarts.

The JsonSerializer class comes from the System.Text.Json namespace and the File class from System.IO. Serialization writes each book's properties as key-value pairs.

Writing with the File class

File.WriteAllText(path, text) writes the whole text to a file at once; it overwrites an existing file or creates a new one. File.ReadAllText(path) reads it back.

The save flow: produce text with Serialize, then write it with WriteAllText. The restore flow: read with ReadAllText, then turn back into an object with Deserialize<List<Book>>.

Exercise Add the Save method

Add a Save(string path) method to the Library class below: it turns _books into text with JsonSerializer.Serialize and writes it with File.WriteAllText. In Main add a book, call Save and print "Saved".

Show example solution

Write public void Save(string path) { string text = JsonSerializer.Serialize(_books); File.WriteAllText(path, text); }. In Main add lib.Save("books.txt"); Console.WriteLine("Saved");
Example solution:
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.Json;

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);
    }

    public void Save(string path)
    {
        string text = JsonSerializer.Serialize(_books);
        File.WriteAllText(path, text);
    }
}

class Program
{
    static void Main()
    {
        var lib = new Library();
        lib.Add(new Book { Title = "Crime and Punishment", Author = "Dostoevsky" });
        lib.Save("books.txt");
        Console.WriteLine("Saved");
    }
}

Test yourself

What does JsonSerializer.Serialize do?
- Turns text into an object
- Deletes a file
- Sorts a list
- Turns an object into text
Show answer

Turns an object into text — Correct. Serialize turns an object into text.

Complete the Serialize call

string text = JsonSerializer.____(_books);

Show answer

string text = JsonSerializer.Serialize(_books);

Match the method to its role

Serialize

Object to text
Deserialize

Text to object
WriteAllText

Write to file
ReadAllText

Read from file

Next lesson: Putting it together

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