var res = new HttpResponseMessage(System.Net.HttpStatusCode.OK);
var ms = new MemoryStream();
res.Content = new StreamContent(ms);
using (var sw = new StreamWriter(ms, System.Text.Encoding.UTF8))
using (var csv = new CsvHelper.CsvWriter(sw))
  csv.WriteRecords(allData.ToList());
return res;

var controller = appContainer().Resolve<MyController>();
  var res = (await controller.Get()) as HttpResponseMessage;
  res.ShouldNotEqual(null);
  var csv = await res.Content.ReadAsStringAsync();

Error while copying content to a stream.
  ----> System.ObjectDisposedException : Cannot access a closed Stream.