WebAPI StreamContent 与 PushStreamContent

Question

我正在实现 BluImp jQuery 文件上传的 MVC4 + WebAPI 版本 在我最初的尝试中一切都运行良好，但我试图确保在下载非常大的文件（~2GB）时充分利用内存。

我已经阅读了 Filip Woj 关于 PushStreamContent 的文章，并尽我所能实现了它（删除异步部分 - 也许这就是问题所在？）。当我运行测试并观看 TaskManager 时，我没有看到内存使用方面有太大差异，并且我试图了解响应处理方式之间的差异。

这是我的 StreamContent 版本：

private HttpResponseMessage DownloadContentNonChunked()
{
    var filename = HttpContext.Current.Request["f"];
    var filePath = _storageRoot + filename;
    if (File.Exists(filePath))
    {
        HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
        response.Content = new StreamContent(new FileStream(filePath, FileMode.Open, FileAccess.Read));
        response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
        response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
        {
            FileName = filename
        };
        return response;
    }
    return ControllerContext.Request.CreateErrorResponse(HttpStatusCode.NotFound, "");
}

这是我的 PushStreamContent 版本：

public class FileDownloadStream
{
    private readonly string _filename;

    public FileDownloadStream(string filePath)
    {
        _filename = filePath;
    }

    public void WriteToStream(Stream outputStream, HttpContent content, TransportContext context)
    {
        try
        {
            var buffer = new byte[4096];

            using (var video = File.Open(_filename, FileMode.Open, FileAccess.Read))
            {
                var length = (int)video.Length;
                var bytesRead = 1;

                while (length > 0 && bytesRead > 0)
                {
                    bytesRead = video.Read(buffer, 0, Math.Min(length, buffer.Length));
                    outputStream.Write(buffer, 0, bytesRead);
                    length -= bytesRead;
                }
            }
        }
        catch (HttpException ex)
        {
            return;
        }
        finally
        {
            outputStream.Close();
        }
    }
}

private HttpResponseMessage DownloadContentChunked()
{
    var filename = HttpContext.Current.Request["f"];
    var filePath = _storageRoot + filename;
    if (File.Exists(filePath))
    {
        var fileDownload = new FileDownloadStream(filePath);
        var response = Request.CreateResponse();
        response.Content = new PushStreamContent(fileDownload.WriteToStream, new MediaTypeHeaderValue("application/octet-stream"));
        response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
        {
            FileName = filename
        };
        return response;
    }
    return ControllerContext.Request.CreateErrorResponse(HttpStatusCode.NotFound, "");
}

我的问题是为什么我没有看到两种方法之间的内存使用有太大差异？此外，我还下载了 StreamContent 类型的 PDB，并且可以看到对缓冲区大小等的引用（见下文），因此我想确切地了解 PushStreamContent 在 StreamContent 之上和之外做了什么。我已经检查了 MSDN 上的类型信息，但这篇文章的解释有点简单！

namespace System.Net.Http
{
  /// <summary>
  /// Provides HTTP content based on a stream.
  /// </summary>
  [__DynamicallyInvokable]
  public class StreamContent : HttpContent
  {
    private Stream content;
    private int bufferSize;
    private bool contentConsumed;
    private long start;
    private const int defaultBufferSize = 4096;

    /// <summary>
    /// Creates a new instance of the <see cref="T:System.Net.Http.StreamContent"/> class.
    /// </summary>
    /// <param name="content">The content used to initialize the <see cref="T:System.Net.Http.StreamContent"/>.</param>
    [__DynamicallyInvokable]
    [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
    public StreamContent(Stream content)
      : this(content, 4096)
    {
    }

Answer 1

关于这两种方法的内存使用情况，对于 StreamContent 和 PushStreamContent，Web API 不会缓冲响应。以下代码快照来自 WebHostBufferPolicySelector。源代码这里.

    /// <summary>
    /// Determines whether the host should buffer the <see cref="HttpResponseMessage"/> entity body.
    /// </summary>
    /// <param name="response">The <see cref="HttpResponseMessage"/>response for which to determine
    /// whether host output buffering should be used for the response entity body.</param>
    /// <returns><c>true</c> if buffering should be used; otherwise a streamed response should be used.</returns>
    public virtual bool UseBufferedOutputStream(HttpResponseMessage response)
    {
        if (response == null)
        {
            throw Error.ArgumentNull("response");
        }

        // Any HttpContent that knows its length is presumably already buffered internally.
        HttpContent content = response.Content;
        if (content != null)
        {
            long? contentLength = content.Headers.ContentLength;
            if (contentLength.HasValue && contentLength.Value >= 0)
            {
                return false;
            }

            // Content length is null or -1 (meaning not known).  
            // Buffer any HttpContent except StreamContent and PushStreamContent
            return !(content is StreamContent || content is PushStreamContent);
        }

        return false;
    }

PushStreamContent还适用于需要将数据“推送”到流中的场景，而StreamContent则从流中“拉取”数据。因此，对于您当前下载文件的场景，使用 StreamContent 应该没问题。

以下示例：

// Here when the response is being written out the data is pulled from the file to the destination(network) stream
response.Content = new StreamContent(File.OpenRead(filePath));

// Here we create a push stream content so that we can use XDocument.Save to push data to the destination(network) stream
XDocument xDoc = XDocument.Load("Sample.xml", LoadOptions.None);
PushStreamContent xDocContent = new PushStreamContent(
(stream, content, context) =>
{
     // After save we close the stream to signal that we are done writing.
     xDoc.Save(stream);
     stream.Close();
},
"application/xml");