Streaming an in-memory generated file in ASP.NET Core

Question

After trawling the internet for hours, I'm lost on how to solve my problem for ASP.NET Core 2.x.

I am generating a CSV on the fly (which can take several minutes) and then trying to send that back to the client. Lots of clients are timing out before I start sending a response, so I am trying to stream the file back to them (with an immediate 200 response) and write to the stream asynchronously. It seemed like this was possible with PushStreamContent previously in ASP, but I'm unsure how to structure my code so the CSV generation is done asynchronously and returning an HTTP response immediately.

[HttpGet("csv")]
public async Task<FileStreamResult> GetCSV(long id)
{
    // this stage can take 2+ mins, which obviously blocks the response
    var data = await GetData(id);
    var records = _csvGenerator.GenerateRecords(data); 

    // using the CsvHelper Nuget package
    var stream = new MemoryStream();
    var writer = new StreamWriter(stream);
    var csv = new CsvWriter(writer);

    csv.WriteRecords(stream, records);
    await writer.FlushAsync();

    return new FileStreamResult(stream, new MediaTypeHeaderValue("text/csv))
    {
        FileDownloadName = "results.csv"
    };
 }

If you make a request to this controller method, you'll get nothing until the whole CSV has finished generating and then you finally get a response, by which point most client requests have timed out.

I've tried wrapping the CSV generation code in a Task.Run() but that has not helped my issue either.

Answer 1

PushStreamContextASP.NET Core 没有内置的类型。但是，您可以构建自己FileCallbackResult的功能相同的东西。此示例代码应执行以下操作：

public class FileCallbackResult : FileResult
{
    private Func<Stream, ActionContext, Task> _callback;

    public FileCallbackResult(MediaTypeHeaderValue contentType, Func<Stream, ActionContext, Task> callback)
        : base(contentType?.ToString())
    {
        if (callback == null)
            throw new ArgumentNullException(nameof(callback));
        _callback = callback;
    }

    public override Task ExecuteResultAsync(ActionContext context)
    {
        if (context == null)
            throw new ArgumentNullException(nameof(context));
        var executor = new FileCallbackResultExecutor(context.HttpContext.RequestServices.GetRequiredService<ILoggerFactory>());
        return executor.ExecuteAsync(context, this);
    }

    private sealed class FileCallbackResultExecutor : FileResultExecutorBase
    {
        public FileCallbackResultExecutor(ILoggerFactory loggerFactory)
            : base(CreateLogger<FileCallbackResultExecutor>(loggerFactory))
        {
        }

        public Task ExecuteAsync(ActionContext context, FileCallbackResult result)
        {
            SetHeadersAndLog(context, result, null);
            return result._callback(context.HttpContext.Response.Body, context);
        }
    }
}

用法：

[HttpGet("csv")]
public IActionResult GetCSV(long id)
{
  return new FileCallbackResult(new MediaTypeHeaderValue("text/csv"), async (outputStream, _) =>
  {
    var data = await GetData(id);
    var records = _csvGenerator.GenerateRecords(data); 
    var writer = new StreamWriter(outputStream);
    var csv = new CsvWriter(writer);
    csv.WriteRecords(stream, records);
    await writer.FlushAsync();
  })
  {
    FileDownloadName = "results.csv"
  };
}

请记住，它FileCallbackResult具有与以下条件相同的限制PushStreamContext：如果在回调中发生错误，则Web服务器没有很好的方式将该错误通知客户端。您所能做的就是传播异常，这将导致ASP.NET提前关闭连接，因此客户端会收到“连接意外关闭”或“下载中止”错误。这是因为在主体开始流式传输之前，HTTP 首先在标头中发送错误代码。