POST IAsyncEnumerable to ASP.NET Core Web API

Question

Can find numerous examples on how return an IAsyncEnumerable<T> from a Web API and consume it in C#, but I can't seem to find any examples on how to send (post) an IAsyncEnumerable<T> to a Web API method.

Is that not possible?

Answer 1

If you are using HttpClient it's not straight-forward to pass an asynchronous stream to it, as none of the existing HttpContent classes support that. The following custom content class can do this:

// only pass the getLength lambda if you are 100% sure the size
public class PullingStreamContent(Func<Stream, CancellationToken, Task> streamWriter, Func<long?>? getLength = null)
    : HttpContent
{
    private readonly Func<Stream, CancellationToken, Task> _streamWriter = streamWriter;
    private readonly Func<long?>? _getLength = getLength;
    
    protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context) =>
        _streamWriter(stream, default);

    protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context, CancellationToken cancellationToken) =>
        _streamWriter(stream, cancellationToken);

    protected override bool TryComputeLength(out long length)
    {
        var l = _getLength?.Invoke();
        length = l.GetValueOrDefault();
        return l.HasValue;
    }
}

Then you can pass an IAsyncEnumerable like this:

private async Task SendMyAsync(CancellationToken cancellationToken)
{
    using var content = new PullingStreamContent(async (outputStream, ct) =>
    {
        IAsyncEnumerable<SomeClass> enumerable = GetSomeAsyncEnumerable(cancellationToken);
        // must be directly of type IAsyncEnumerable not a derived type
        await _jsonSerializer.SerializeAsync(outputStream, enumerable, cancellationToken: ct);
    });
    content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

    using var request = new HttpRequestMessage(HttpMethod.Post, "https://someUrl");
    request.Content = upstreamContent;
    using var response = await _httpClient.SendAsync(upstreamRequest, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
    response.EnsureSuccessStatusCode();
    using var responseStream = await response.ReadAsStreamAsync(cancellationToken);
    // do stuff with response
}