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)
        }
)