Question

我尝试利用WebAPI将大量数据从SQL Server流式传输到分隔文件，而无需等待整个结果集从数据库返回。我对Async和Await非常可怕，并没有把我的头包裹起来。看来我应该使用PushStreamContent和SqlDataReader异步方法做什么。
这是我的DataAccessLayer：

   Public Async Function ExecuteToResponseStream(command As SqlCommand, responseStream As IO.Stream) As Threading.Tasks.Task
      Using responseStream
         Dim newConString As String = _conString
         Using con As SqlConnection = New SqlConnection(newConString)
            Await con.OpenAsync()
            'Add params
            command.Connection = con

            Using ms As IO.MemoryStream = New IO.MemoryStream
               Using ts As IO.StreamWriter = New IO.StreamWriter(ms)

                  Using reader As SqlDataReader = Await command.ExecuteReaderAsync(CommandBehavior.SequentialAccess)
                     While Await reader.ReadAsync()
                        Dim loValues As New List(Of String)
                        For i = 0 To reader.FieldCount - 1
                           loValues.Add(reader(i).ToString)
                        Next
                        Await ts.WriteAsync(String.Join(vbTab, loValues.ToArray))
                        Await ms.CopyToAsync(responseStream)
                     End While
                  End Using
               End Using
            End Using
         End Using
      End Using
   End Function

我的存储库有一个签名为：函数ExportAsync（id As String，responseStream As IO.Stream）As Task

最后，WebAPI客户端：

   Public Function Export(id As String) As HttpResponseMessage
      Dim resp As New HttpResponseMessage(HttpStatusCode.OK)
      Dim onStreamAvailable As Action(Of IO.Stream, HttpContent, TransportContext) = Async Sub(responseStream, content, context)
        Await repository.ExportAsync(rc, responseStream)
     End Sub

      resp.Content = New PushStreamContent(onStreamAvailable)
      resp.Content.Headers.ContentType = New Headers.MediaTypeHeaderValue("application/octet-stream")
      resp.Content.Headers.ContentDisposition = New Headers.ContentDispositionHeaderValue("attachment")
      resp.Content.Headers.ContentDisposition.FileName = "Somefilename"

      Return resp
   End Function

客户端函数似乎正确退出并最终返回响应。对浏览器的最终响应确实看起来像一个附件，它是空的，我吮吸流，但似乎它不是以那种神奇的异步方式发生的。

另外，在我看来，这是一个有点简单的练习，虽然我无法在网上找到一个好的实现。也许我真的错了。

Answer 1

我发现了一些曾经/不起作用的东西。

<强>更新

存储库方法

Public Async Function ExportToCSV(inputParameters, stream As IO.Stream) As Threading.Tasks.Task
   'Build SqlCommand from inputParameters
   Await sqlClient.ReaderToDelimitedAsync(","c, _SqlCommand, stream)
End Function

自定义SqlClient包装器方法

   Async Function ReaderToDelimitedAsync(delimiter As Char, command As SqlCommand, stream As IO.Stream) As Threading.Tasks.Task
  Try

     Await command.Connection.OpenAsync
     Using rdr As SqlDataReader = Await command.ExecuteReaderAsync
        Using sw As New IO.StreamWriter(stream, Text.Encoding.GetEncoding("Windows-1252"))
           Dim los As New List(Of String)
           For i = 0 To rdr.FieldCount - 1
              los.Add(String.Format("{0}{1}{0}", ControlChars.Quote, rdr.GetName(i)))
           Next
           Await sw.WriteLineAsync(String.Join(delimiter, los.ToArray))
           Await sw.FlushAsync

           While Await rdr.ReadAsync
              Dim loVals As New List(Of String)
              For i = 0 To rdr.FieldCount - 1
                 Dim strVal As String = rdr.GetValue(i).ToString
                 strVal = strVal.Replace(ControlChars.Quote, Chr(39)).Trim(Chr(10), Chr(13), Chr(32)) '.Replace(Chr(13), " ").Replace(Chr(10), " ")
                 strVal = strVal.Replace(vbCrLf + vbCrLf, vbCrLf).Replace(vbCrLf + vbCrLf, vbCrLf)
                 strVal = strVal.Substring(0, Math.Min(strVal.Length, 32000))
                 loVals.Add(String.Format("{0}{1}{0}", ControlChars.Quote, strVal))
              Next
              Await sw.WriteLineAsync(String.Join(","c, loVals.ToArray))
              Await sw.FlushAsync()
           End While

        End Using
     End Using
  Catch ex As Exception
     Throw ex
  End Try
   End Function

使用它：

  <HttpPost, ActionName("Download"), ValidateAntiForgeryHeader(False)> Public Function DownloadReport(postparameters) As HttpResponseMessage

      Dim resp As New HttpResponseMessage(HttpStatusCode.OK)

         resp.Content = New PushStreamContent(Function(stream, http_content, transportcontextcontext)
                                                 Return repository.Export(postparameters, stream)
                                              End Function, "text/csv")
         resp.Content.Headers.ContentType = New Headers.MediaTypeHeaderValue("text/csv")
     resp.Content.Headers.ContentDisposition = New Headers.ContentDispositionHeaderValue("attachment")
      resp.Content.Headers.ContentDisposition.FileName = reporttemplate.Filename

      Return resp

   End Function

我也尝试过：

   <HttpPost, ActionName("Download"), ValidateAntiForgeryHeader(False)> Public Function DownloadReport(postparameters) As HttpResponseMessage

      Dim resp As New HttpResponseMessage(HttpStatusCode.OK)

         resp.Content = New PushStreamContent(Async Function(stream, http_content, transportcontextcontext)
                                                 Await repository.Export(postparameters, stream)
                                              End Function, "text/csv")
         resp.Content.Headers.ContentType = New Headers.MediaTypeHeaderValue("text/csv")
     resp.Content.Headers.ContentDisposition = New Headers.ContentDispositionHeaderValue("attachment")
      resp.Content.Headers.ContentDisposition.FileName = reporttemplate.Filename

      Return resp

   End Function

问题是在SqlDataReader关闭（End Using语句）之前，下载不会在客户端启动。我的理解是PushStreamContent应该在遇到它的第一次Await时返回，随后对FlushAsync的调用应该开始将流传递给客户端。我仍然很难理解这个Async Await模式。有什么想法吗？

PushStreamContent使用SqlDataReader.ExecuteReaderAsync将CSV或分隔文件流式传输到客户端

1 个答案: