Can I annotate model classes so they are automatically included in a source-generated JsonSerializerContext?

[tsctsc6]

Hi, I am using .NET (AOT) with the System.Text.Json source generator.
Right now, to enable static reflection (source-generated serialization), I have to manually list every DTO/model type inside a custom JsonSerializerContext, for example:

[JsonSerializable(typeof(UserDto))]
[JsonSerializable(typeof(ProductDto))]
[JsonSerializable(typeof(OrderDto))]
public partial class MyJsonSerializerContext : JsonSerializerContext;

This becomes hard to maintain as the number of models grows, because I have to write all DTO/model at the same place while all DTO/model distributed in various files.

Is there a way to annotate each model class directly so that the generator automatically adds it to my MyJsonSerializerContext? For example something like:

[JsonIncludeInContext(typeof(MyJsonSerializerContext))]
public class UserDto
{
}

And then the source generator would automatically include it in the specified context.

The ways I thought of

Method 1

Declare partial class MyJsonSerializerContext at in various files:

[JsonSerializable(typeof(OrderDto))]
public partial class MyJsonSerializerContext : JsonSerializerContext;

[JsonSerializable(typeof(ProductDto))]
public partial class MyJsonSerializerContext : JsonSerializerContext;

However, it does not work, I got some compiler errors:

• "MyJsonSerializerContext" is not include "Default".
• "MyJsonSerializerContext" is not impl "GetTypeInfo".
• "AppJsonSerializerContext" is not impl "JsonSerializerContext.GeneratedSerializerOptions.get".
• The required parameter "options" for "JsonSerializerContext.JsonSerializerContext(JsonSerializerOptions?)" was not provided.

Method 2

One class corresponds to one JsonSerializerContext.

However, I still have to regester all JsonSerializerContexts, the situation has not changed substantially.

What I am looking for

• A built-in attribute that works like JsonIncludeInContext mentioned above
  OR
• A code pattern that declare the DTO/model's JsonSerializerContext in the distributed file with the same JsonSerializerContext
  OR
• Confirmation that this is currently impossible by design

Environment

• .NET 10

Thanks!

[mjodheim]

Short answer: there is no built-in model-side attribute for this in .NET 10.

[JsonSerializable] is context-scoped: it must be applied to a class deriving from JsonSerializerContext, not directly to a DTO. The source generator needs a compile-time closed set of root types. Types referenced by the members of those roots are discovered automatically, except for runtime types hidden behind members declared as object.

Your first method is valid C#, but on .NET 10 it hits a known System.Text.Json source-generator bug: dotnet/runtime#99669.

When [JsonSerializable] attributes are placed on multiple partial declarations of the same context, the generator is triggered once per declaration while seeing the combined attributes from every partial. It then tries to emit the same generated files more than once. Generation aborts, and the missing Default, GetTypeInfo, and GeneratedSerializerOptions errors are cascading errors from that failure.

This was fixed by dotnet/runtime#123417, merged into main on January 27, 2026. The fix selects one canonical partial declaration while collecting the attributes from every partial. However, it is not present in the current release/10.0 branch, so on .NET 10 the reliable options are:

1. Keep all [JsonSerializable] attributes on one partial declaration:

[JsonSerializable(typeof(UserDto))]
[JsonSerializable(typeof(ProductDto))]
[JsonSerializable(typeof(OrderDto))]
internal partial class MyJsonSerializerContext : JsonSerializerContext;

2. Split the models into several feature-specific contexts and combine them:

using System.Text.Json;
using System.Text.Json.Serialization;
using System.Text.Json.Serialization.Metadata;

[JsonSerializable(typeof(UserDto))]
internal partial class UserJsonContext : JsonSerializerContext;

[JsonSerializable(typeof(ProductDto))]
internal partial class ProductJsonContext : JsonSerializerContext;

[JsonSerializable(typeof(OrderDto))]
internal partial class OrderJsonContext : JsonSerializerContext;

var options = new JsonSerializerOptions
{
    TypeInfoResolver = JsonTypeInfoResolver.Combine(
        UserJsonContext.Default,
        ProductJsonContext.Default,
        OrderJsonContext.Default)
};

Once using a System.Text.Json version that contains #123417, the distributed partial-context pattern should work:

// UserDto.cs
[JsonSerializable(typeof(UserDto))]
internal partial class MyJsonSerializerContext : JsonSerializerContext;

// ProductDto.cs
[JsonSerializable(typeof(ProductDto))]
internal partial class MyJsonSerializerContext;

For .NET 10, though, there is no built-in DTO annotation that automatically registers the model in a chosen context. Implementing that behavior would require a custom source generator that produces either the central context declaration or the resolver registration.