Using the .NET JSON Source Generator with ASP.NET Core Minimal APIs

How to use the new .NET 6 JSON source generator in an application using ASP.NET Core Minimal APIs.

28 November 2021 by Martin Costello |
28 November 2021 by Martin Costello

I've recently completed upgrading a bunch of personal and work applications to ASP.NET Core 6, and now that the dust has finally settled on those efforts, I thought I'd look into a new feature of .NET 6 that I hadn't tried out yet - JSON source generators.

If you haven't come across them before, C# source generators are a way to write some code that can generate more code during compilation. It's a form of metaprogramming.

One of the benefits of the new JSON source generator for the System.Text.Json serializer is that it is more performant that the APIs introduced as part of .NET 5. This is because the serializer is able to leverage code that is compiled ahead-of-time (the source generator part) to serialize and deserialize objects to and from JSON without using reflection (which is relatively slow).

It sounds like that could give applications a performance boost at runtime, but how can we use the new JSON source generator with ASP.NET Core Minimal APIs?

Out-of-the box, ASP.NET Core Minimal APIs do not currently support using the JSON source generator and just use the JsonSerializer.SerializeAsync() methods under-the-hood (this code, which calls through to this code). For us to leverage the JSON source generator we need to be able to use one of the new SerializeAsync() overloads added as part of .NET 6 that take either a JsonSerializerContext or a JsonTypeInfo<T> parameter somehow. How can we achieve that while still keeping our HTTP endpoint code minimal?

One approach we can use to do this is to use the extensibility hook built into the Results class - the Results.Extensions property and the IResultExtensions interface. By adding a custom extension method for the IResultExtensions interface, we can add a new Json() method we can use in our Minimal API endpoints which we can use to leverage the JSON source generator.

Here's a simplified example of this for JsonTypeInfo<T>:

using System.Text.Json.Serialization;
using System.Text.Json.Serialization.Metadata;
using Microsoft.AspNetCore.Http.Result;

namespace Microsoft.AspNetCore.Http;

public static class ResultExtensions
    public static IResult Json<T>(
        this IResultExtensions extensions,
        T value,
        JsonTypeInfo<T> jsonTypeInfo,
        string? contentType = null,
        int? statusCode = null)
        return new JsonResult<T>
            ContentType = contentType,
            JsonTypeInfo = jsonTypeInfo,
            StatusCode = statusCode,
            Value = value,

We extend the IResultExtensions and pass through the JsonTypeInfo<T> along with the data we want to serialize as JSON and assign them to a JsonResult<T> instance, which is returned to the caller. This is a custom implementation of the IResult interface, which is where we'll put the code to actually do the JSON serialization.

Using ASP.NET Core 6's own implementation as inspiration, we can then write some extension methods for the HttpResponse class which can then pass this information along to the JsonSerializer class to serialize our objects to JSON using our source-generated implementation.

Below are some snippets from the relevant code.

// JsonResult<T>
Task IResult.ExecuteAsync(HttpContext httpContext)
    if (StatusCode is int statusCode)
        httpContext.Response.StatusCode = statusCode;

    return httpContext.Response.WriteAsJsonAsync(Value, JsonTypeInfo, ContentType);

// HttpResponseJsonExtensions
public static Task WriteAsJsonAsync<T>(
    this HttpResponse response,
    T value,
    JsonTypeInfo<T> jsonTypeInfo,
    string? contentType,
    CancellationToken cancellationToken = default)
    response.ContentType = contentType ?? "application/json; charset=utf-8";
    return JsonSerializer.SerializeAsync(response.Body, value, jsonTypeInfo, cancellationToken);

With these extensions available, we can then modify our Minimal API endpoints to use our new Json() method with our source-generated code.

var planets = new Planet[]
    new() { Name = "Mercury" },
    new() { Name = "Venus" },
    new() { Name = "Earth" },
    new() { Name = "Mars" }

// Create an instance of our custom JsonSerializerContext using the JSON
// serializer settings we want to use with it, such as to be in camelCase.
var context = new StellarJsonSerializerContext(new(JsonSerializerDefaults.Web));

// Use Results.Extensions.Json() with our serializer context
app.MapGet("/planets", () => Results.Extensions.Json(planets, context.PlanetArray));

public class Planet
    public string Name { get; set; }

// Our custom JSON serializer context that generates code to serialize
// arrays of planets so that we can use it with our HTTP endpoint.
public partial class StellarJsonSerializerContext : JsonSerializerContext

We could also add other extension methods that we can use that rely on a JsonSerializerContext being registered with the service provider used for dependency injection to simplify the endpoint code even further. Then the context isn't even referenced in the endpoints' code, and the only difference to using the built-in Json() method is the Extensions part, like this.

app.MapGet("/planets", () => Results.Extensions.Json(planets));

A complete sample application containing all the code referenced in this blog post can be found in GitHub here:

I hope you find it useful - happy coding!