C# Tutorial · Advanced C# · Lesson 21 of 22

C# Files and JSON

Read and write files with File, and serialize JSON with System.Text.Json.

Files

The static File class covers most needs: ReadAllText, WriteAllText, ReadAllLines, AppendAllText, Exists, Delete. Use Path.Combine to build paths.

JSON

System.Text.Json is built in:

Example 1

using System.Text.Json;

string path = Path.Combine(Path.GetTempPath(), "notes.txt");
File.WriteAllText(path, "first\n");
File.AppendAllLines(path, ["second", "third"]);
string[] lines = File.ReadAllLines(path);
Console.WriteLine($"{lines.Length} lines, last: {lines[^1]}");
File.Delete(path);
Console.WriteLine(File.Exists(path));

var options = new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase };
var product = new Product(1, "Mug", 9.5m, ["kitchen"]);
string json = JsonSerializer.Serialize(product, options);
Console.WriteLine(json);
var back = JsonSerializer.Deserialize<Product>("""{"id":2,"name":"Lamp","price":45,"tags":[]}""", options);
Console.WriteLine($"{back!.Name} costs {back.Price}, {back.Tags.Count} tags");

record Product(int Id, string Name, decimal Price, List<string> Tags);

Output

3 lines, last: third
False
{"id":1,"name":"Mug","price":9.5,"tags":["kitchen"]}
Lamp costs 45, 0 tags

Run this on your computer — see “Get Started” for setup.

Test yourself

Which namespace contains the built-in JSON serializer?

Newtonsoft.Json only
System.Text.Json
System.Json.Serializer
Microsoft.Json

Show answer

Answer: System.Text.Json