Reflection versus source generation in System.Text.Json

This article explains the differences between reflection and source generation as it relates to System.Text.Json serialization. It also provides guidance on how to choose the best approach for your scenario.

Metadata collection

To serialize or deserialize a type, JsonSerializer needs information about how to access the members of the type. JsonSerializer needs the following information:

• How to access property getters and fields for serialization.
• How to access a constructor, property setters, and fields for deserialization.
• Information about which attributes have been used to customize serialization or deserialization.
• Run-time configuration from JsonSerializerOptions.

This information is referred to as metadata.

Reflection

By default, JsonSerializer collects metadata at run time by using reflection. Whenever JsonSerializer has to serialize or deserialize a type for the first time, it collects and caches this metadata. The metadata collection process takes time and uses memory.

Source generation

As an alternative, System.Text.Json can use the C# source generation feature to improve performance, reduce private memory usage, and facilitate assembly trimming, which reduces app size. In addition, certain reflection APIs can't be used in Native AOT applications, so you must use source generation for those apps.

Source generation can be used in two modes:

• Metadata-based mode

  During compilation, System.Text.Json collects the information needed for serialization and generates source code files that populate JSON contract metadata for the requested types.

• Serialization-optimization (fast path) mode

  JsonSerializer features that customize the output of serialization, such as naming policies and reference preservation, carry a performance overhead. In serialization-optimization mode, System.Text.Json generates optimized serialization code that uses Utf8JsonWriter directly. This optimized or fast path code increases serialization throughput.

  Fast-path deserialization isn't currently available. For more information, see dotnet/runtime issue 55043.

Source generation for System.Text.Json requires C# 9.0 or a later version.

Feature comparison

Choose reflection or source-generation modes based on the following benefits that each one offers:

Benefit	Reflection	Source generation (Metadata-based mode)	Source generation (Serialization-optimization mode)
Simpler to code.	✔️	❌	❌
Simpler to debug.	❌	✔️	✔️
Supports non-public members.	✔️	✔️*	✔️*
Supports all available serialization customizations.	✔️	❌†	❌†
Reduces start-up time.	❌	✔️	✔️
Reduces private memory usage.	❌	✔️	✔️
Eliminates run-time reflection.	❌	✔️	✔️
Facilitates trim-safe app size reduction.	❌	✔️	✔️
Increases serialization throughput.	❌	❌	✔️

* The source generator supports some non-public members, for example, internal types in the same assembly. † Source-generated contracts can be modified using the contract customization API.