📝 Lesson 4.2: JSON Serialization

What Is Serialization?

Serialization is converting an object in memory into a format that can be stored or transmitted — usually text. Deserialization is the reverse: rebuilding the object from that text. JSON (JavaScript Object Notation) is the most common format for this.

A C# object and its JSON look like mirror images of each other:

.NET's built-in JSON tools live in System.Text.Json. (You may need using System.Text.Json; at the top.)

Serializing to JSON

JsonSerializer.Serialize turns any object into a JSON string. It automatically includes the object's public properties:

using System.Text.Json;

record Product(string Name, string Category, decimal Price);

Product keyboard = new Product("Keyboard", "Electronics", 45m);

string json = JsonSerializer.Serialize(keyboard);
Console.WriteLine(json);
// {"Name":"Keyboard","Category":"Electronics","Price":45}

Collections serialize just as easily — a List becomes a JSON array:

List<Product> products = new List<Product>
{
    new Product("Keyboard", "Electronics", 45m),
    new Product("Desk", "Furniture", 150m)
};

string json = JsonSerializer.Serialize(products);
Console.WriteLine(json);
// [{"Name":"Keyboard",...},{"Name":"Desk",...}]

Deserializing from JSON

JsonSerializer.Deserialize<T> rebuilds an object from JSON text. You tell it the target type with a generic argument (Lesson 1.2), and it fills in the properties by matching names:

string json = "{\"Name\":\"Mouse\",\"Category\":\"Electronics\",\"Price\":25}";

Product? product = JsonSerializer.Deserialize<Product>(json);

Console.WriteLine(product?.Name);    // Mouse
Console.WriteLine(product?.Price);   // 25

Deserializing a JSON array back into a List:

string json = "[{\"Name\":\"Keyboard\",\"Category\":\"Electronics\",\"Price\":45}]";

List<Product>? items = JsonSerializer.Deserialize<List<Product>>(json);
Console.WriteLine(items?.Count);     // 1

Options: Formatting & Naming

By default the JSON is compact and uses your C# property names (PascalCase). Pass a JsonSerializerOptions to change that — two options you'll use constantly:

var options = new JsonSerializerOptions
{
    WriteIndented = true,                                  // pretty-print
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase     // camelCase keys
};

string json = JsonSerializer.Serialize(keyboard, options);
Console.WriteLine(json);

{
  "name": "Keyboard",
  "category": "Electronics",
  "price": 45
}

Saving Objects to a File

Now combine this lesson with the last one: serialize to a JSON string, then write it to a file with the File class. This is a complete "save and load" for structured data.

using System.Text.Json;

var options = new JsonSerializerOptions { WriteIndented = true };
string path = "products.json";

// SAVE: object → JSON → file
List<Product> products = new List<Product>
{
    new Product("Keyboard", "Electronics", 45m),
    new Product("Desk", "Furniture", 150m)
};
string json = JsonSerializer.Serialize(products, options);
File.WriteAllText(path, json);
Console.WriteLine("Saved!");

// LOAD: file → JSON → object
if (File.Exists(path))
{
    string loaded = File.ReadAllText(path);
    List<Product>? restored = JsonSerializer.Deserialize<List<Product>>(loaded);
    Console.WriteLine($"Loaded {restored?.Count} products.");
}

Common Gotchas

Exercise & Quiz

using System.Text.Json;

record TaskItem(string Title, bool IsDone);

var tasks = new List<TaskItem>
{
    new TaskItem("Learn JSON", true),
    new TaskItem("Build an app", false)
};

string path = "tasks.json";
var options = new JsonSerializerOptions { WriteIndented = true };

// TODO: serialize tasks and write to path
// TODO: read path, deserialize, and print each task

using System.Text.Json;

record TaskItem(string Title, bool IsDone);

var tasks = new List<TaskItem>
{
    new TaskItem("Learn JSON", true),
    new TaskItem("Build an app", false)
};

string path = "tasks.json";
var options = new JsonSerializerOptions { WriteIndented = true };

// Save
File.WriteAllText(path, JsonSerializer.Serialize(tasks, options));
Console.WriteLine("Saved tasks.json");

// Load
try
{
    string text = File.ReadAllText(path);
    var loaded = JsonSerializer.Deserialize<List<TaskItem>>(text);

    foreach (var t in loaded ?? new List<TaskItem>())
    {
        string mark = t.IsDone ? "[x]" : "[ ]";
        Console.WriteLine($"{mark} {t.Title}");
    }
}
catch (JsonException ex)
{
    Console.WriteLine($"Invalid JSON: {ex.Message}");
}

Saved tasks.json
[x] Learn JSON
[ ] Build an app

Summary

📚 Additional Resources

• System.Text.Json overview — Microsoft Docs
• How to serialize and deserialize JSON
• JsonSerializer — API reference

🚀 What's Next?

You can save and load structured data locally. But modern apps also fetch data over the internet — often as JSON. In Lesson 4.3: Asynchronous Programming, you'll learn async/await, the key to doing slow work (like network and file access) without freezing your app.