Zero-Allocation JSON Parsing with `Utf8JsonReader` & `Utf8JsonWriter`

1Concept

`Utf8JsonReader` is a high-speed `ref struct` that parses raw UTF-8 byte streams directly into spans without converting them to UTF-16 C# strings, achieving 10x lower memory overhead than traditional JSON parsers.

2Architecture Diagram

Raw UTF-8 Bytes: [ 0x7B, 0x22, 0x69, 0x64, 0x22, ... ] ("{"id":101}")
       │
       ▼
 [ Utf8JsonReader (Ref Struct on Stack) ]
       ├── Token: StartObject
       ├── Token: PropertyName -> "id" (Read as ReadOnlySpan<byte>)
       └── Token: Number -> 101 (Zero String Allocations!)

3Code Example

using System;
using System.Text;
using System.Text.Json;

public class Utf8JsonReaderDemo
{
    public static void Main()
    {
        byte[] jsonBytes = Encoding.UTF8.GetBytes("{\"metric\": \"cpu_usage\", \"value\": 94.5}");
        var reader = new Utf8JsonReader(jsonBytes);

        while (reader.Read())
        {
            if (reader.TokenType == JsonTokenType.PropertyName && reader.ValueTextEquals("value"u8))
            {
                reader.Read();
                double value = reader.GetDouble();
                Console.WriteLine($"Parsed Metric Value (Zero UTF-16 String Allocation): {value}");
            }
        }
    }
}

4Expected Output

Parsed Metric Value (Zero UTF-16 String Allocation): 94.5

5Key Takeaways

• ✓`"value"u8` (UTF-8 string literal in C# 11+) provides compile-time zero-allocation UTF-8 byte spans.
• ✓`Utf8JsonReader` is non-allocating and stack-allocated (`ref struct`).
• ✓Used internally by `System.Text.Json` for maximum throughput.