Try the new System.Text.Json source generator

Layomi Akinrinade


In .NET 6.0, we are shipping a new C# source generator to help improve the performance of applications that use System.Text.Json. In this post, I’ll go over why we built it, how it works, and what benefits you can experience in your application.

With the introduction of the System.Text.Json source generator, we now have a few models for JSON serialization in .NET to choose from, using JsonSerializer. There is the existing model which is backed by runtime reflection, and two new compile-time source generation modes; where the generator generates optimized serialization logic, a static data access model, or both. In both source-generation scenarios, the generated artifacts are passed directly to JsonSerializer as a performance optimization. Here’s an overview of the functionality that each serialization model provides:

JsonSerializer JsonSerializer + pre-generating optimized serialization logic JsonSerializer + pre-generating data access model
Increases serialization throughput No (baseline) Yes No
Reduces start-up time No (baseline) Yes Yes
Reduces private memory usage No (baseline) Yes Yes
Eliminates runtime reflection No Yes Yes
Facilitates trim-safe app size reduction No Yes Yes
Supports all serialization features Yes No Yes

Getting the source generator

The source generator can be used in any .NET C# project, including console applications, class libraries, web, and Blazor applications. You can try out the source generator by using the latest build of the System.Text.Json NuGet package. Starting with the upcoming .NET 6.0 Preview 7 this won’t be necessary when targeting net6.0.

The source generator is compatible with other target framework monikers (TFMs) aside from .NET 6.0, that is .NET 5.0 and lower, .NET Framework, and .NET Standard. The API shape of the generated source code is consistent across the TFMs, but the implementation may vary based on the framework APIs that are available on each TFM.

The minimum version of the .NET SDK required for C# source generation is .NET 5.0.

What is the System.Text.Json source generator?

The backbone of nearly all .NET serializers is reflection. Reflection provides great capability for certain scenarios, but not as the basis of high-performance cloud-native applications (which typically (de)serialize and process a lot of JSON documents). Reflection is a problem for startup, memory usage, and assembly trimming.

An alternative to runtime reflection is compile-time source generation. Source generators generate C# source files that can be compiled as part of the library or application build. Generating source code at compile time can provide many benefits to .NET applications, including increased performance. In .NET 6, we are including a new source generator as part of System.Text.Json. The JSON source generator works in conjunction with JsonSerializer, and can be configured in multiple ways. Existing JsonSerializer functionality will continue to work as-is, so it’s your decision whether you use the new source generator. The approach the JSON source generator takes to provide these benefits is to move the runtime inspection of JSON-serializable types to compile-time, where it generates a static model to access data on the types, optimized serialization logic using Utf8JsonWriter directly, or both.

To opt into source generation, you can define a partial class which derives from a new class called JsonSerializerContext, and indicate serializable types using a new JsonSerializableAttribute class.

For example, given a simple Person type to serialize:

namespace Test
    internal class Person
        public string FirstName { get; set; }
        public string LastName { get; set; }

We would specify the type to the source generator as follows:

using System.Text.Json.Serialization;

namespace Test
    internal partial class MyJsonContext : JsonSerializerContext

As part of the build, the source generator will augment the MyJsonContext partial class with the following shape:

internal partial class MyJsonContext : JsonSerializerContext
    public static MyJsonContext Default { get; }

    public JsonTypeInfo<Person> Person { get; }

    public MyJsonContext(JsonSerializerOptions options) { }

    public override JsonTypeInfo GetTypeInfo(Type type) => ...;

The generated source code can be integrated into the compiling application by passing it directly to new overloads on JsonSerializer:

Person person = new() { FirstName = "Jane", LastName = "Doe" };
byte[] utf8Json = JsonSerializer.SerializeToUtf8Bytes(person, MyJsonContext.Default.Person);
person = JsonSerializer.Deserialize(utf8Json, MyJsonContext.Default.Person):

Why do source generation?

JsonSerializer is a tool that serializes .NET objects into JSON strings, and deserializes .NET objects from JSON strings. To process a type, the serializer needs information about how to access its members. When serializing, the serializer needs access to the object’s property and field getters. Similarly, when deserializing, the serializer needs access to a constructor to instantiate the type, and also to setters for the object’s properties and fields.

System.Text.Json exposes mechanisms for influencing serialization and deserialization behavior when using JsonSerializer, via JsonSerializerOptions (which allows runtime configuration), and also via attributes like [JsonPropertyName(string)] and [JsonIgnore] (which allow design-time configuration). When serializing and deserializing instances of a type, the serializer needs information about this configuration so that it can be honored.

When processing JSON-serializable types, the serializer needs information about object members and feature configuration in a structured, optimized format. We can refer to this structured information as the serialization metadata required to serialize a type.

In previous versions of System.Text.Json, serialization metadata could only be computed at runtime, during the first serialization or deserialization routine of every type in any object graph passed to the serializer. After this metadata is generated, the serializer performs the actual serialization and deserialization. The result of this computation is cached for reuse during subsequent JSON processing routines. The generation phase is based on reflection, and is computationally expensive both in terms of time and allocations. We can refer to this phase as the serializer’s “warm-up” phase.

The System.Text.Json source generator helps us to remove this warm-up phase by shifting the runtime inspection of serializable types using reflection to compile-time. The result of this inspection can be source code that initializes instances of structured serialization metadata. The generator can also generate highly-optimized serialization logic that can honor a set of serialization features that are specified ahead-of-time. By default, the generator emits both kinds of source, but can be configured to generate only one of these kinds of outputs either across a set of types, or per serializable type.

This generated metadata is included in the compiled assembly, where it can be initialized and passed directly to JsonSerializer so that the serializer doesn’t have to generate it at runtime. This helps reduce the costs of the first serialization or deserialization of each type. With these characteristics, using the source generator can provide the following benefits to applications that use System.Text.Json:

  • Increased serialization throughput
  • Reduced start-up time
  • Reduced private memory usage
  • Removed runtime use of System.Reflection and System.Reflection.Emit
  • Trim-compatible serialization which reduces application size

Introducing JsonTypeInfo<T>, JsonTypeInfo, and JsonSerializerContext

Implementations of the JsonTypeInfo<T>, JsonTypeInfo, and JsonSerializerContext types are the primary result of JSON source generation.

The JsonTypeInfo<T> type contains structured information about how to serialize and deserialize a single type. This information can contain metadata about how to access its members. This information is required when the serializer itself is performing the (de)serialization of the type, using the robust logic it has to support all the features that can be configured with JsonSerializerOptions or serialization attributes. This includes advanced features like async (de)serialization and reference handling. When only a limited set of features are needed, JsonTypeInfo<T> can contain optimized, pre-generated serialization logic (using Utf8JsonWriter directly) which the serializer can invoke instead of going through its own code-paths. Invoking this logic can lead to substantial serialization throughput improvements. A JsonTypeInfo<T> instance is tightly bound to a single instance of JsonSerializerOptions.

The JsonTypeInfo type provides an untyped abstraction over JsonTypeInfo<T>. JsonSerializer utilizes it to retrieve a JsonTypeInfo<T> instance from a JsonSerializerContext instance via JsonSerializerContext.GetTypeInfo.

The JsonSerializerContext type contains JsonTypeInfo<T> instances for multiple types. In addition to helping JsonSerializer retrieve instances of JsonTypeInfo<T> via JsonSerializerContext.GetTypeInfo, it also provides a mechanism for initializing all the type metadata using specific JsonSerializerOptions instances provided by users.

Source generation modes

The System.Text.Json source generator has two modes: one that generates type-metadata initialization logic, and another that generates serialization logic. Users can configure the source generator to use one or both of these modes for JSON-serializable types in a project, depending on the (de)serialization scenario. Metadata generated for a type contains structured information in a format that can be optimally utilized by the serializer to serialize and deserialize instances of that type to and from JSON representations. Serialization logic uses direct calls to Utf8JsonWriter methods to write JSON representations of .NET objects, using a predetermined set of serialization options. By default, the source generator generates both metadata initialization logic and serialization logic, but can be configured to generate just one type of logic. To set the generation mode for the entire context (set of serializable types), use JsonSourceGenerationOptionsAttribute.GenerationMode, while to set the mode for a specific type, use JsonSerializableAttribute.GenerationMode.

Generating optimized serialization logic

The first source generation mode we’ll look at is JsonSourceGenerationMode.Serialization. It delivers much higher performance than using existing JsonSerializer methods by generating source code that uses Utf8JsonWriter directly. In short, source generators offer a way of giving you a different implementation at compile-time in order to make the runtime experience better.

JsonSerializer is a powerful tool that has many features that can influence the (de)serialization of .NET types from/into the JSON format. It is fast, but can have some performance overhead when only a subset of features are needed for a serialization routine. Going forward, we will update JsonSerializer and the new source generator together. Sometimes, a new JsonSerializer feature will have accompanying support for optimized serialization logic and sometimes not, depending on how feasible it is to generate logic to support the feature.

Given our Person type from above, the source generator can be configured to generate serialization logic for instances of the type, given some pre-defined serializer options. Note that the class name MyJsonContext is arbitrary. You can use whatever class name you want.

using System.Text.Json.Serialization;

namespace Test
        PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
        GenerationMode = JsonSourceGenerationMode.Serialization)]
    internal partial class MyJsonContext : JsonSerializerContext

We have defined a set of JsonSerializer features that are supported in this mode via JsonSourceGenerationOptionsAttribute. As shown above, these features can be specified to the source generator ahead of time, to avoid extra checks at runtime.

As part of the build, the source generator augments the MyJsonContext partial class with the same shape shown above.

The serializer invocation with this mode could look like the following example. This example allows us to trim out a lot of the System.Text.Json implementation since we don’t call into JsonSerializer.

using MemoryStream ms = new();
using Utf8JsonWriter writer = new(ms);

MyJsonContext.Default.Person.Serialize(writer, new Person { FirstName = "Jane", LastName = "Doe" });

// Writer contains:
// {"firstName":"Jane","lastName":"Doe"}

Alternatively, you can continue to use JsonSerializer, and instead pass an instance of the generated code to it, with MyJsonContext.Default.Person.

JsonSerializer.Serialize(person, MyJsonContext.Default.Person);

Here’s a similar use, with a different overload.

JsonSerializer.Serialize(person, typeof(Person), MyJsonContext.Default);

The difference between these two overloads is that the first is using the typed metadata implementation — JsonTypeInfo<T> — and the second one is using a more general JsonSerializerContext implementation that does type tests to determine if a typed implementation exists. It is a little slower (due to the type tests), as a result. If there is no source-generated implementation for a given type, then the serializer throws a NotSupportedException. It does not fallback to a reflection-based implementation (as an explicit design choice).

In the example above, the MyJsonContext.Default.Person property returns a JsonTypeInfo<Person>. The Default property returns a MyJsonContext instance whose backing JsonSerializerOptions instance matches the values set by the JsonSourceGenerationOptionsAttribute set on the JsonSerializerContext. If the attribute was not present, a default JsonSerializerOptions instance (i.e the result of new JsonSerializerOptions(JsonSerializerDefaults.General)) would be used.

This fastest and most optimized source generation mode — based on Utf8JsonWriter — is currently only available for serialization. Similar support for deserialization — based on Utf8JsonReader — will be considered for support in a future version of .NET. You will see in the following sections that there are other modes that benefit deserialization.

Generating type-metadata initialization logic

The generator can be configured to generate type-metadata initialization logic — with the JsonSourceGenerationMode.Metadata mode — instead of the complete serialization logic. This mode provides a static data access model for the regular JsonSerializer code paths to invoke when executing serialization and deserailization logic. This mode is useful if you need features that are not supported by the JsonSourceGenerationMode.Serialization mode, such as reference handling and async serialization. This mode also provide benefits for deserialization, which the serialization-logic mode does not.

The JsonSourceGenerationMode.Metadata mode provides much of the benefits of source generation, with the exception of improved serialization throughput. With this mode, runtime metadata generation is shifted to compile-time. Like the previous mode, the required metadata is generated into the compiling assembly, where it can be initialized and passed to JsonSerializer. This approach reduces the costs of the first serialization or deserialization of each type. Most of the other benefits stated earlier also apply, such as lower memory usage.

This mode is configured in a similar way as the previous example, except that we don’t specify feature options ahead of time, and we specify different generation mode:

using System.Text.Json.Serialization;

namespace Test
    [JsonSerializable(typeof(Person), GenerationMode = JsonSourceGenerationMode.Metadata)]
    internal partial class MyJsonContext : JsonSerializerContext

The generator augments the partial context class with the same shape as shown earlier.

The serializer invocation with this mode would look like the following example.

JsonSerializerOptions options = new()
    ReferenceHander = ReferenceHandler.Preserve,
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase

// Use your custom options to initialize a context instance.
MyJsonContext context = new(options);

string json = JsonSerializer.Serialize(person, context.Person);

// {"id":1,"firstName":"Jane","lastName":"Doe"}

Using this mode, you should see a significant performance improvement while enjoying the full range of (de)serializer features. This mode is a nice middle-ground option, depending on your needs.

Generating both serialization logic and metadata initialization logic

The default configuration for the source generation mode is JsonSourceGenerationMode.Default. This is an “everything on” mode that enables both of the source generator modes that were just covered. For example, you might only need features compatible with JsonSourceGenerationMode.Serialization for serialization, but also want to improve deserialization performance. In this scenario, you should see faster initial serialization and deserialization, faster serialization throughput, lower memory use, and decreased app size with assembly trimming.

You’d configure the generator as follows. Note you don’t have specify a generation mode since the default is to generate both kinds of logic.

using System.Text.Json.Serialization;

namespace Test
    [JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
    internal partial class MyJsonContext : JsonSerializerContext

Again, the generated API shape remains the same. We can use the generated source as follows:

// Serializer invokes pre-generated serialization logic for increased throughput and other benefits.
string json = JsonSerializer.Serialize(person, MyJsonContext.Default.Person);

// Serializer uses pre-generated type-metadata and avoids warm-up stage for deserialization, alongside other benefits.
Person person = JsonSerializer.Deserialize(json, MyJsonContext.Default.Person);

New APIs on JsonSerializer and in System.Net.Http.Json

In addition to the new APIs we’ve gone over to configure the source generator, we’ve added APIs to consume it’s output. We’ve already seen some of the new APIs in the examples above, where we pass JsonTypeInfo<T> and JsonSerializerContext instances directly to JsonSerializer as a performance optimization. We’ve also added more overloads to JsonSerializer and also in the System.Net.Http.Json APIs which help optimize the processing of JSON data when interacting with HttpClient and JsonContent.

New JsonSerializer APIs

namespace System.Text.Json
    public static class JsonSerializer
        public static object? Deserialize(ReadOnlySpan<byte> utf8Json, Type returnType, JsonSerializerContext context) => ...;
        public static object? Deserialize(ReadOnlySpan<char> json, Type returnType, JsonSerializerContext context) => ...;
        public static object? Deserialize(string json, Type returnType, JsonSerializerContext context) => ...;
        public static object? Deserialize(ref Utf8JsonReader reader, Type returnType, JsonSerializerContext context) => ...;
        public static ValueTask<object?> DeserializeAsync(Stream utf8Json, Type returnType, JsonSerializerContext context, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static ValueTask<TValue?> DeserializeAsync<TValue>(Stream utf8Json, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static TValue? Deserialize<TValue>(ReadOnlySpan<byte> utf8Json, JsonTypeInfo<TValue> jsonTypeInfo) => ...;
        public static TValue? Deserialize<TValue>(string json, JsonTypeInfo<TValue> jsonTypeInfo) => ...;
        public static TValue? Deserialize<TValue>(ReadOnlySpan<char> json, JsonTypeInfo<TValue> jsonTypeInfo) => ...;
        public static TValue? Deserialize<TValue>(ref Utf8JsonReader reader, JsonTypeInfo<TValue> jsonTypeInfo) => ...;
        public static string Serialize(object? value, Type inputType, JsonSerializerContext context) => ...;
        public static void Serialize(Utf8JsonWriter writer, object? value, Type inputType, JsonSerializerContext context) { }
        public static Task SerializeAsync(Stream utf8Json, object? value, Type inputType, JsonSerializerContext context, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task SerializeAsync<TValue>(Stream utf8Json, TValue value, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static byte[] SerializeToUtf8Bytes(object? value, Type inputType, JsonSerializerContext context) => ...;
        public static byte[] SerializeToUtf8Bytes<TValue>(TValue value, JsonTypeInfo<TValue> jsonTypeInfo) => ...;
        public static void Serialize<TValue>(Utf8JsonWriter writer, TValue value, JsonTypeInfo<TValue> jsonTypeInfo) { }
        public static string Serialize<TValue>(TValue value, JsonTypeInfo<TValue> jsonTypeInfo) => ...;

New System.Net.Http.Json APIs

namespace System.Net.Http.Json
    public static partial class HttpClientJsonExtensions
        public static Task<object?> GetFromJsonAsync(this HttpClient client, string? requestUri, Type type, JsonSerializerContext context, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<object?> GetFromJsonAsync(this HttpClient client, System.Uri? requestUri, Type type, JsonSerializerContext context, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<TValue?> GetFromJsonAsync<TValue>(this HttpClient client, string? requestUri, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<TValue?> GetFromJsonAsync<TValue>(this HttpClient client, System.Uri? requestUri, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<HttpResponseMessage> PostAsJsonAsync<TValue>(this HttpClient client, string? requestUri, TValue value, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<HttpResponseMessage> PostAsJsonAsync<TValue>(this HttpClient client, System.Uri? requestUri, TValue value, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<HttpResponseMessage> PutAsJsonAsync<TValue>(this HttpClient client, string? requestUri, TValue value, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<HttpResponseMessage> PutAsJsonAsync<TValue>(this HttpClient client, System.Uri? requestUri, TValue value, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
    public static partial class HttpContentJsonExtensions
        public static Task<object?> ReadFromJsonAsync(this HttpContent content, Type type, JsonSerializerContext context, CancellationToken cancellationToken = default(CancellationToken)) => ...;
        public static Task<T?> ReadFromJsonAsync<T>(this HttpContent content, JsonTypeInfo<TValue> jsonTypeInfo, CancellationToken cancellationToken = default(CancellationToken)) => ...;
    public sealed partial class JsonContent : HttpContent
        public static JsonContent<object?> Create(object? inputValue, Type inputType, JsonSerializerContext context, MediaTypeHeaderValue? mediaType = null) => ...;
        public static JsonContent<TValue> Create<TValue>(TValue? inputValue, JsonTypeInfo<TValue> jsonTypeInfo, MediaTypeHeaderValue? mediaType = null) => ...;
    public sealed partial class JsonContent<TValue> : JsonContent
        public TValue? Value { get }

How source generation provides benefits

Improved serialization throughput

Pre-generating and using optimized serialization logic that honors only features needed in an app leads to increased performance over using JsonSerializer‘s robust serialization logic. The serializer supports all features, which means there’s more logic to tear through during serialization, which can show up during measurements.

Serializing POCOs

Given our Person type, we can observe that serialization is ~1.62x faster when using the source generator.

Method Mean Error StdDev Ratio RatioSD Gen 0 Gen 1 Gen 2 Allocated
Serializer 243.1 ns 4.83 ns 9.54 ns 1.00 0.00
SrcGenSerializer 149.3 ns 2.04 ns 1.91 ns 0.62 0.03

Serializing collections

Using the same Person type, we observe significant performance boosts when serializing arrays with different lengths, all while not allocating at all.

Method Count Mean Error StdDev Ratio Gen 0 Gen 1 Gen 2 Allocated
Serializer 10 2,392.5 ns 17.42 ns 13.60 ns 1.00 0.0801 344 B
SrcGenSerializer 10 989.4 ns 6.74 ns 5.62 ns 0.41
Serializer 100 21,427.1 ns 189.33 ns 167.84 ns 1.00 0.0610 344 B
SrcGenSerializer 100 10,137.3 ns 125.61 ns 111.35 ns 0.47
Serializer 1000 215,102.4 ns 1,737.91 ns 1,356.85 ns 1.00 344 B
SrcGenSerializer 1000 104,970.5 ns 345.48 ns 288.49 ns 0.49

TechEmpower caching benchmark

The TechEmpower caching benchmark exercises a platform or framework’s in-memory caching of information sourced from a database. Following the benchmark specification, we perform JSON serialization of the cached data in order to send them as a response to the test harness. With source generation, we’ve been able to significantly increase our performance in this benchmark.

Requests/sec Requests
net5.0 243,000 3,669,151
net6.0 260,928 3,939,804
net6.0 + JSON source gen 364,224 5,499,468

We observe a ~100K RPS gain, which is a ~40% increase.

Faster startup & reduced private memory

Moving retrieval of type metadata from runtime to compile-time means that there is less work for the serializer to do on start-up, which leads to a reduction in the amount of time it takes to perform the first serialization or deserialization of each type.

In previous versions of System.Text.Json, the serializer always used Reflection.Emit where possible to generate fast member accessors to constructors, properties, and fields. Generating these IL methods takes a non-trivial amount of time, but also consumes private memory. With source generators, we are able to generate code that that statically invokes these accessors. This eliminates time and allocation cost due to reflection.

Similarly, all serialization and deserialization of JSON data was was performed within JsonConverter<T> instances. The serializer would statically initialize several built-in converter instances to provide default functionality. User applications paid the cost of these allocations, even when only a few of these converters are needed given the input object graphs. With source generation, we can initialize only the converters that are needed by the types indicated to the generator (in the JsonSourceGeneration.Metadata mode), and also skip the use of converters entirely (in the JsonSourceGeneration.Serialization mode).

Using a simple Message type:

public class JsonMessage
    public string message { get; set; }

We can observe startup improvements during serialization and deserialization.


Elapsed time (ms) Allocated (KB)
Serializer 28.25 1110.00
SrcGenSerializer 12.75 563.00


Elapsed time (ms) Allocated (KB)
Serializer 24.75 1457.00
SrcGenSerializer 15.50 1025.00

We see that that it takes substantially less time and lower allocations to perform the first (de)serialization for the type.

Trim correctness

By eliminating runtime reflection, we avoid the primary coding pattern that is unfriendly to ILLinker analysis. Given that the reflection-based code is trimmed out, applications that use System.Text.Json go from having several ILLinker analysis warnings when trimming to having absolutely none. This means that applications that use the System.Text.Json source generator can be safely trimmed, provided there are no other warnings due to the user app itself, or other parts of the BCL.

Reduced app size

By inspecting serializable types at compile-time instead of at runtime, two major things are done to reduce the size of the consuming application. First, we can detect which custom or built-in JsonConverter<T> types are needed in the application at runtime, and reference them statically in the generated metadata. This allows the ILLinker to trim out JSON converter types which will not be needed in the application at runtime. Similarly, inspecting input types at compile-time eliminates the need to do so at runtime. This removes the need for lots of System.Reflection APIs at runtime, so the ILLinker can trim code internal code in System.Text.Json which uses with those APIs. Unused source code further in the dependency graph is also trimmed out.

Size reductions are based on which JsonSerializer methods are used. If the new APIs that take pre-generated metadata are used, then you may observe size reductions in your app.

Using a simple console program that does a roundtrip serialization and deserialization of our Message type using JSON source generation, we can observe a size reduction after trimming, when compared to the same program which does not use source generation.

App size (MB) System.Text.Json.dll size (KB)
Serializer 21.7 989
SrcGenSerializer 20.8 460

Using source generated code in ASP.NET Core


In Blazor applications, pre-generated logic for serializable types can be forwarded to the serializer directly via the new APIs being added in the System.Net.Http.Json namespace. For example, to asynchronously deserialize a list of weather forecast objects from an HttpClient, you can use a new overload on the HttpClient.GetFromJsonAsync method:

internal partial class MyJsonContext : JsonSerializerContext { }

@code {
    private WeatherForecast[] forecasts;

    private static JsonSerializerOptions Options = new(JsonSerializerDefaults.Web);
    private static MyJsonContext Context = new MyJsonContext(Options);

    protected override async Task OnInitializedAsync()
        forecasts = await Http.GetFromJsonAsync("sample-data/weather.json", Context.WeatherForecastArray);

MVC, WebAPIs, SignalR, Houdini

These ASP.NET Core services can retrieve user-generated contexts with already-existing APIs to configure serialization options.

internal partial class MyJsonContext : JsonSerializerContext { }

services.AddControllers().AddJsonOptions(options => options.AddContext<MyJsonContext>());

In the future, these services can expose APIs to directly take JsonTypeInfo<T> or JsonSerializerContext instances.

Performing JSON serialization on behalf of others

Similar to the ASP.NET Core scenarios above, library or framework developers can provide new APIs that accept JsonSerializerOptions or JsonSerializerContext instances to forward to the serializer on behalf of the user.


You might be wondering why the output of the source generator is passed directly to JsonSerializer via new overloads, rather than say initialized implicitly on application start up and registered in a global cache where the serializer can retrieve it. One benefit is for app trimming. By providing dedicated entry-points for source-generated serializers and data access models, we are statically providing the serializer with all the information that it needs to serialize input types, which allows it to shed lots of unused code and dependencies (both reflection-related and otherwise). Another benefit is that it provides an opportunity for callers to consider versioning, that is whether the code generated for serializable types is in sync with the serializable types themselves. This is an important consideration when using source generation, particulaly for serialization.

Having source-generator output that does not match the corresponding serializable types can cause serious issues, including having data being accidentally included or excluded from serialization. These issues could be really hard to diagnose. The source generator design avoids patterns which can cause versioning issues such as app-global registration of generated artifacts. This sort of global registration can cause contention issues between serialization settings across different assemblies. Each assembly might use different versions of the source generator, potentially resulting in apps having different behavior in development and in production, depending on which set of generated artifacts (among the implementations in various assemblies) are selected by the serializer.


Improving the performance of applications that use System.Text.Json has been a continuous process, and a primary goal of the library since its inception. Compile-time source generation has helped us increase performance and develop a new model of trim-safe serialization. We’d like you to try the source generator and give us feedback on how it affects the perf in your app, any usability issues, and any bugs you may find.

Community contributions are always welcome. If you’d like to contribute to System.Text.Json, check out our list of up-for-grabs issues on GitHub.


Leave a comment

  • Avatar
    Max Mustermueller

    So this is the only thing you got for .NET 6 and nobody asked for?! Polymorphic support has been requested since .NET 3 and is now moved to .NET 7. There are many others high voted requested for JSON existing since more than 2 years now, which also have been moved to .NET 7. Why do you ignore the community/developers?

    • Vincent Thorn
      Vincent Thorn

      Because they don’t give a damn about you! 🙂 VS and C# is not a “tool to simplify programmer’s life”. It’s PRODUCT TO SELL. More fancy features they do (even useless), more they can write in advert booklet. Working with Git inside VS? I never need it, I use Mercurial. But no, they WILL waste resources to implement it! Even if nobody use it in IDE (I prefer well designed external tools). This is sample of MS thinking: “how to sell it”, not “how to make it better”.

    • Avatar
      Jonathan Allen

      Because that’s hard and boring. They want to work on cool stuff, not the basics.

      This is also why EF still doesn’t have real support for stored procedures. They just want to play with new tech, not do the things we actually need the most.

    • Avatar
      Immo LandwerthMicrosoft employee

      Why do you ignore the community/developers?

      We aren’t ignoring developers. System.Text.Json is simply an area that has had a lot of feature requests. We have addressed many of them. Don’t take my word for it, go to GitHub and check out what we merged.

      At the same time, we also need to keep pushing the platform forward. This often means laying the foundation for new technologies. Static linking is on the rise, especially for mobile workloads, but developers increasingly want this capability for other workloads too. Source generating serializers is a part in this.

      Contrary to popular belief, the big features that are announced for a given release often started a few releases before, where we had to do small incremental features across the board. So we can’t just use ALL resources and only work on features customers requested for a given release, we also need to invest into areas that customers haven’t asked yet, but for which we have good evidence that they will ask for soon. This is crucial for products that need to scale to an ever changing landscape of usage scenarios and requirements.

      Our planning is in the open. We have specifically put a goal in for .NET 6 to address more requests for JSON, which we did. Did we finish all of them? No. But I don’t think not being able to afford all of them is the same as ignoring requests.

    • Avatar
      Aigner D.

      I think you did not fully read the post or you did not understand it otherwise you would not claim that “nobody asked for it”, because this is a very awesome and important step.

      Also polymorphic support was already possible in the very first version with some workarounds: Change the type to object (not so nice, but works) or keep the class type but use a custom converter which can be implemented as a generic class, so you only have to do it once and it’s only a few lines of codes and will work for all polymorphic uses cases you need. You don’t even have to implement it yourself, because there are several examples online.

      Also to the other guy: You have to realize that they implement the framework for a huge amount of users and not just for you, just because you don’t need (or understand?) a feature does not mean that is is useless or wasted resources.

  • Vincent Thorn
    Vincent Thorn

    Easy recognizable MS style: take simplistic problem and pile up a lot of API around. Why the hell you need all of those functions?!! You can EASY speed-up serialization just by making hidden static class which is generated at compile time and TRANSPARENTLY does the job. Why intervention of programmer is needed?

  • Avatar
    Petr Houška

    Looks great!

    Do I understand it correctly (seems to be the way according to examples) that JsonSerializerContext is thread-safe?

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    Jonathan Allen

    Why are you wasting time with this when you still haven’t gotten around to implementing support for the DataContract/DataMember attributes so we can port our code from Json.NET?

    All this new tech sure is shiny, but we have hundreds of DTOs that have to be rewritten because you haven’t finish the basics yet.

    • Layomi Akinrinade
      Layomi AkinrinadeMicrosoft employee

      We’ve messaged before that System.Text.Json is not a drop-in replacement for Newtonsoft.Json, and also that System.Runtime.Serialization attributes are considered legacy systems that we do not want to support natively in the serializer. We are however, working toward allowing users to easily detect them with minimal configuration code, and set the corresponding type metadata to influence (de)serialization.

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    Shawn Van Ness

    This is so critical for Azure Functions and AWS Lambda etc.. where the dollar cost is a function of CPU-time and RAM-usage, and it’s stateless middleware so 90% of what these functions do is de/serialize JSON, in and out at both ends. Reflection is needlessly expensive, and error-prone. Can’t wait to try this!