JsonHttpContent 与 ASP.NET 核心 3.1

Question

I created a custom HttpContent to efficiently send objects using System.Text.Json with HttpClient. Sadly there's very little documentation around how to correctly do this. The following class results in an error about malformed JSON data at my test endpoint. Looking at the packets in wireshark shows that only an empty json object ( {} ) is transmitted. I also tried to flush the stream (same result) or dispose it after writing to it (this results in an error about tring to access a disposed object).

using System.IO;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Text.Json;
using System.Threading.Tasks;

namespace Common
{
    public sealed class JsonContent : HttpContent
    {
        private readonly object _content;
        private readonly JsonSerializerOptions _options;

        public JsonContent(object content, JsonSerializerOptions options)
        {
            _content = content;
            _options = options;
            Headers.ContentType = new MediaTypeHeaderValue("application/json")
            {
                CharSet = Encoding.UTF8.WebName
            };
        }

        protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
        {
            return JsonSerializer.SerializeAsync(stream, _content, _options);
        }

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

The whole thing is used like this:

public static async Task<HttpResponseMessage> PostAsJsonAsync<T>(this HttpClient client, string url, T content,
    CancellationToken ct = default) where T : class
{
    return await client.PostAsync(url, new JsonContent(content, Json.DefaultSerializerOptions), ct);
}

What am I missing here?

Answer 1

Seemingly, you have missed the type of _content . Try this:

protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
{
    return JsonSerializer.SerializeAsync(stream, _content, _content.GetType(), _options);
}

The method you called was actually SerializeAsync<TValue>(...) , which uses typeof(TValue) as the type to serialize. Because your _content is an instance of object , so TValue in this case is object , thus _content was serialized as object . That is why you see only {} .

The fix calls another overload SerializeAsync(...) which allows you to specify the correct type to serialize.

Answer 2

Does your object contain fields or properties.. System.Text.Json will not serialize fields..