C# Async ApiController 过早关闭 OutputStream

Question

今天的问题是，当使用 WebApi 2 和基于 Async ApiController 的 Get 方法时，它会返回文件的内容。当我将 Get 方法更改为同步时，它工作得很好，但是一旦我将它转换回异步，它就会过早地关闭流。 （提琴手报告连接被中止）工作同步代码是：

 public void Get(int id)
    {
        try
        {
            FileInfo fileInfo = logic.GetFileInfoSync(id);
            HttpResponse response = HttpContext.Current.Response;
            response.Clear();
            response.ClearContent();
            response.Buffer = true;
            response.AddHeader("Content-Disposition", "attachment; filename=\"" + fileInfo.Node.Name + fileInfo.Ext + "\"");
            response.AddHeader("Content-Length", fileInfo.SizeInBytes.ToString());
            response.ContentType = "application/octet-stream";
            logic.GetDownloadStreamSync(id, response.OutputStream);
            response.StatusCode = (int)HttpStatusCode.OK;
            //HttpContext.Current.ApplicationInstance.CompleteRequest();
             response.End();
        }
        catch(Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }
    }

而GetDownloadStreamSync如下：

public async Task GetDownloadStream(string fileIdentifier, Stream streamToCopyTo)
{
    string filePath = Path.Combine(fileIdentifierFolder, fileIdentifier);
    using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None, BufferSize, false))
    {
        fs.CopyTo(streamToCopyTo);
    }
 }

--------异步代码----------

异步版本完全相同，除了：

public async Task Get(int id)
{
    FileInfo fileInfo = await logic.GetFileInfoSync(id); // database opp
            HttpResponse response = HttpContext.Current.Response;
            response.Clear();
            response.ClearContent();
            response.Buffer = true;
            response.AddHeader("Content-Disposition", "attachment; filename=\"" + fileInfo.Node.Name + fileInfo.Ext + "\"");
            response.AddHeader("Content-Length", fileInfo.SizeInBytes.ToString());
            response.ContentType = "application/octet-stream";
            await logic.GetDownloadStreamSync(id, response.OutputStream); 
                           //database opp + file I/O
            response.StatusCode = (int)HttpStatusCode.OK;
             //HttpContext.Current.ApplicationInstance.CompleteRequest();
             response.End();
}

GetDownloadStream 的异步实现如下：（streamToCopyTo 是来自 response.OutputStream 的 OutputStream）

    public async Task GetDownloadStream(string fileIdentifier, Stream streamToCopyTo)
{
    using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None, BufferSize, true))
    {
        await fs.CopyToAsync(streamToCopyTo);
    }
}

我们正在尝试从头到尾采用 async/await 模式，所以希望有人知道为什么这会失败？我也尝试过不调用 Response.End()、Response.Flush() 和 HttpContext.Current.ApplicationInstance.CompleteRequest()。此外，为了响应下面的问题/cmets，我在 response.End() 上放置了一个断点，结果它没有被 GetDownloadStream 方法命中。也许 OutputStream 不是异步的？欢迎任何想法！ 谢谢

****************************** 最终解决方案 ******************* ********

非常感谢所有发表评论的人，尤其感谢 @Noseratio 对 FileOptions.DeleteOnClose 的建议。

[HttpGet]
public async Task<HttpResponseMessage> Get(long id)
{
        HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
        Node node = await logic.GetFileInfoForNodeAsync(id);

        result.Content = new StreamContent(await logic.GetDownloadStreamAsync(id));
        result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
        result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
        {
            FileName = node.Name + node.FileInfo.Extension
        };
        result.Content.Headers.ContentLength = node.FileInfo.SizeInBytes;
        return result
}

GetDownloadStreamAsync 看起来像这样：

 FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None, BufferSize, FileOptions.DeleteOnClose | FileOptions.Asynchronous);

我忽略了我也在动态解密文件流，这确实有效，所以对于那些感兴趣的人......

FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None, BufferSize, FileOptions.DeleteOnClose | FileOptions.Asynchronous);
RijndaelManaged rm = new RijndaelManaged();
return new CryptoStream(fs, GetDecryptor(rm, password), CryptoStreamMode.Read);

Answer 1

需要一个完整的复制案例来回答您的确切问题，但我认为您根本不需要 async/await。我还认为您应该尽可能避免直接使用HttpContext.Current.Response，尤其是在异步 WebAPI 控制器方法中。

在这种特殊情况下，您可以使用HttpResponseMessage:

[HttpGet]
public HttpResponseMessage Get(int id)
{
    HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
    FileInfo fileInfo = logic.GetFileInfoSync(id);

    FileStream fs = new FileStream(
        filePath, FileMode.Open, FileAccess.Read, FileShare.None, BufferSize, false);

    result.Content = new StreamContent(fs);
    result.Content.Headers.ContentType = 
        new MediaTypeHeaderValue("application/octet-stream");
    result.Content.Headers.ContentDisposition = 
        new ContentDispositionHeaderValue("attachment") 
        {
            FileName = fileInfo.Node.Name + fileInfo.Ext
        };
    result.Content.Headers.ContentLength = fileInfo.SizeInBytes;

    return result;
}

这里没有显式的异步，所以方法不是async。但是如果你还需要引入一些await，方法应该是这样的：

[HttpGet]
public async Task<HttpResponseMessage> Get(int id)
{
    HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
    // ...
    await fs.CopyToAsync(streamToCopyTo)
    // ...
    return result;
}

Answer 2

你问题的根源其实在于Response.End()的使用。当您运行 Async 时，它将在完成流式传输文件内容之前执行 Response.End()。使用 Sync 版本时不会出现这种情况，因为 Response.End() 在完成流式传输文件内容之后才会被调用。

Response.End() 是一种非常糟糕的方式来表示您已完成处理，因为它会引发 TreadAbortException。相反，您应该使用HttpContext.Current.ApplicationInstance.CompleteRequest()

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