A class for plugging the JSON source generator with Refit

SystemTextJsonContentSerializerForSourceGenerator.cs

using System.Net;
using System.Reflection;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace Refit;

/// <summary>
/// A class representing an implementation of <see cref="IHttpContentSerializer"/> that can be
/// used with the <c>System.Text.Json</c> source generators. This class cannot be inherited.
/// </summary>
/// <param name="jsonSerializerContext">The <see cref="JsonSerializerContext"/> to use.</param>
public sealed class SystemTextJsonContentSerializerForSourceGenerator(JsonSerializerContext jsonSerializerContext) : IHttpContentSerializer
{
    //// Based on https://github.com/reactiveui/refit/blob/main/Refit/SystemTextJsonContentSerializer.cs

    private readonly JsonSerializerContext _jsonSerializerContext = jsonSerializerContext;

    /// <inheritdoc />
    public HttpContent ToHttpContent<T>(T item)
    {
        ArgumentNullException.ThrowIfNull(item);

#if NET8_0_OR_GREATER
        var jsonTypeInfo = _jsonSerializerContext.GetTypeInfo(typeof(T));
        return System.Net.Http.Json.JsonContent.Create(item, jsonTypeInfo);
#else
        return new JsonContent(item, typeof(T), _jsonSerializerContext);
#endif
    }

    /// <inheritdoc />
    public async Task<T?> FromHttpContentAsync<T>(HttpContent content, CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(content);

        using var stream = await content.ReadAsStreamAsync(cancellationToken).ConfigureAwait(false);

#if NET8_0_OR_GREATER
        var jsonTypeInfo = _jsonSerializerContext.GetTypeInfo(typeof(T)) as JsonTypeInfo<T>;
        return await JsonSerializer.DeserializeAsync(stream, jsonTypeInfo, cancellationToken).ConfigureAwait(false);
#else
        object result = await JsonSerializer.DeserializeAsync(
            stream,
            typeof(T),
            _jsonSerializerContext,
            cancellationToken).ConfigureAwait(false);

        return (T?)result;
#endif
    }

    /// <inheritdoc />
    public string? GetFieldNameForProperty(PropertyInfo propertyInfo)
    {
        ArgumentNullException.ThrowIfNull(propertyInfo);

        return propertyInfo
            .GetCustomAttributes<JsonPropertyNameAttribute>(true)
            .Select(x => x.Name)
            .FirstOrDefault();
    }

#if !NET8_0_OR_GREATER
    /// <summary>
    /// Based on https://github.com/dotnet/runtime/blob/main/src/libraries/System.Net.Http.Json/src/System/Net/Http/Json/JsonContent.cs.
    /// </summary>
    private sealed class JsonContent : HttpContent
    {
        private readonly object _value;
        private readonly Type _objectType;
        private readonly JsonSerializerContext _serializerContext;

        internal JsonContent(object inputValue, Type inputType, JsonSerializerContext context)
        {
            _value = inputValue;
            _objectType = inputType;
            _serializerContext = context;
            Headers.ContentType = new("application/json") { CharSet = "utf-8" };
        }

        protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context)
        {
            return JsonSerializer.SerializeAsync(stream, _value, _objectType, _serializerContext, CancellationToken.None);
        }

        protected override bool TryComputeLength(out long length)
        {
            length = 0;
            return false;
        }
    }
#endif
}

Thanks for this example! I tried it out with net8.0 and replaced ToHttpContent like:

public HttpContent ToHttpContent<T>(T item)
{
    ArgumentNullException.ThrowIfNull(item);
    return JsonContent.Create(item, typeof(T), options: _jsonSerializerContext.Options);
}

And deleted the JsonContent implementation. Then used it like:

services.AddRefitClient<IGitLabApi>(
    new RefitSettings
        {
            ContentSerializer =
                new SystemTextJsonContentSerializerForSourceGenerator(GitlabJsonContext.Default)
        }
)

@martincostello Would you be able to give me an example of your JsonSerializerContext?

Currently running into problems after migrating my app from Xamarin.iOS to .NET for iOS with my API calls using Refit. I've already created my own JsonSerializerContext sub-class, but adding a JsonSerializable attribute for every type that Refit needs to serialize/deserialize to, feels very obnoxious to me. Am I missing something?

I've stopped using Refit in my own code as it's not AoT-friendly due to its use of Reflection, but you can find examples of my JsonSerializerContext classes with this GitHub code search link.

You'll need to declare as many types with attributes as are required for the source generator to be able to recursively go through the definition(s) to find every type as that's how the JSON source generator is designed to work as it needs to know ahead of time what you're going to need at runtime to generate the appropriate code. For example, if you had a class that had a property of every other type you (de)serialize, then you would only need to add an attribute for that type as it would by extension have to include those types to be able to handle that parent class. Similarly, if you serialize a T[], then you only need to add [JsonSerializabletypeof(T[])] as by extension that includes typeof(T).