CSystemTextJson

CSystemTextJson, commonly referred to as System.Text.Json, is a high-performance JSON library for the .NET platform. It provides APIs to read, write, deserialize, and serialize JSON data with a focus on speed and low memory usage. It is part of the base class library introduced with .NET Core 3.0.

Core API access is centered on JsonSerializer for high-level serialization and deserialization, and on streaming helpers such as Utf8JsonReader and Utf8JsonWriter for low-level access. For in-memory DOM-style inspection, it offers JsonDocument and JsonElement. Configuration is performed through JsonSerializerOptions, which cover property naming policies, case sensitivity, and custom converters, among other settings.

Performance and adoption are hallmarks of System.Text.Json. The library is optimized for speed and reduced allocations, making it well suited to high-throughput scenarios. It is the default JSON provider in ASP.NET Core applications and integrates tightly with the framework’s hosting and serialization pipelines. In recent .NET releases, features such as source generation have further boosted serialization performance by generating code at compile time.

Features and capabilities include support for custom converters, the JsonInclude attribute to include non-public members, and options to ignore null values. Reference handling and polymorphic deserialization are available through runtime options and converters in newer runtimes. The library also supports advanced customization while maintaining type safety, and it can be extended with source-generated serializers for specific types to improve performance.

History and scope: System.Text.Json was introduced with .NET Core 3.0 as a built-in alternative to Json.NET for many scenarios. Over successive Framework releases it gained capabilities such as reference handling, improved polymorphism, and optional source generation, broadening its applicability. It remains the recommended default for many new .NET applications, although some projects may still rely on Newtonsoft.Json for feature parity or compatibility.