PushStreamContent.cs

class PushStreamContent : HttpContent
{
    private readonly Func<Stream, Task> _pushAction;

    public PushStreamContent(Func<Stream, Task> pushAction)
    {
        _pushAction = pushAction;
    }

    protected override async Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        await _pushAction(stream);
    }
    
    protected override bool TryComputeLength(out long length)
    {
        length = -1;
        return false;
    }
}