Make SystemTextJsonInputFormatter IAsyncEnumerable aware

[cmeyertons]

In ASP.NET Core 6, preventing buffering of output results was completed

Can we achieve the same benefit on the Input / Request side as well via the SystemTextJsonInputFormatter?

[captainsafia]

Triage: this work would depend on adding support for Stream parameters in MVC actions, see #41540.

[cmeyertons]

@captainsafia im sure there’s aspects I’m not considering, but we did get this working inserting a new InputFormatter into the pipeline.

Would it be helpful to post the approach here?

[CheloXL]

@captainsafia im sure there’s aspects I’m not considering, but we did get this working inserting a new InputFormatter into the pipeline.

Would it be helpful to post the approach here?

I would like to see your approach, as currently I'm doing that by reading the Body in the action method and would like to have an argument in the method instead...

[cmeyertons]

I am on vacation now away from my computer, back on April 6th. Can post example then!

[cmeyertons]

@CheloXL As promised (a day late :) )

using Microsoft.AspNetCore.Mvc.Formatters;
using Microsoft.Net.Http.Headers;
using System.Text;
using System.Text.Json;

namespace Example;

internal class AsyncEnumerableJsonInputFormatter : TextInputFormatter, IInputFormatterExceptionPolicy
{
    private static readonly Type _asyncEnumerableType = typeof(IAsyncEnumerable<>);

    public InputFormatterExceptionPolicy ExceptionPolicy => InputFormatterExceptionPolicy.MalformedInputExceptions;

    public JsonSerializerOptions SerializerOptions { get; }

    public AsyncEnumerableJsonInputFormatter()
    {
        SerializerOptions = new JsonSerializerOptions(JsonSerializerDefaults.Web);

        SupportedEncodings.Add(UTF8EncodingWithoutBOM);

        SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/json"));
    }

    protected override bool CanReadType(Type type)
    {
        return type.IsGenericType && type.GetGenericTypeDefinition() == _asyncEnumerableType;
    }

    public override Task<InputFormatterResult> ReadRequestBodyAsync(InputFormatterContext context, Encoding encoding)
    {
        var modelReaderType = typeof(AsyncEnumerableStreamReader<>).MakeGenericType(context.ModelType.GetGenericArguments()[0]);
        var modelReader = (IAsyncEnumerableStreamReader)Activator.CreateInstance(modelReaderType)!;

        var result = InputFormatterResult.Success(modelReader.ReadModel(context.HttpContext.Request.Body, SerializerOptions));

        return Task.FromResult(result);
    }

    private interface IAsyncEnumerableStreamReader
    {
        object ReadModel(Stream inputStream, JsonSerializerOptions serializerOptions);
    }


    private class AsyncEnumerableStreamReader<T> : IAsyncEnumerableStreamReader
    {
        public object ReadModel(Stream inputStream, JsonSerializerOptions serializerOptions)
        {
            return JsonSerializer.DeserializeAsyncEnumerable<T>(inputStream, serializerOptions);
        }
    }
}

Bind to your MvcOptions during AddMvc (or calling Configure<MvcOptions>, etc)

builder.Services.AddMvc(options =>
{
    options.InputFormatters.Insert(0, new AsyncEnumerableJsonInputFormatter())
});

Controller methods now can accept IAsyncEnumerable<T> directly from the body

[CheloXL]

Excellent, thank you very much!