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，但一旦我将其转换回异步，它就会过早地关闭流。（Fiddler 报告连接已中止）工作的同步代码是：

 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();
}

使用 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会失败？我也试过不调用 Response.End()、Response.Flush() 和 HttpContext.Current.ApplicationInstance.CompleteRequest()。此外，为了响应下面的 questions/comments，我在 response.End() 上放置了一个断点，结果是它没有被击中到 GetDownloadStream 方法已经完成。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，尤其是@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 如下所示：

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

我忘记了我也在即时解密文件流，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

你问题的根源实际上在于Response.End()的使用。当您运行异步时，它会在完成流式传输文件内容之前执行 Response.End()。使用同步版本时看不到这一点，因为 Response.End() 在文件内容流式传输完成后才会被调用。

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

有关详细信息，请参阅本文Response.End, Response.Close, and How Customer Feedback Helps Us Improve MSDN Documentation

Answer 2

需要一个完整的重现案例才能回答您的确切问题，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;
}

这里没有明确的异步，所以方法不是async。但是如果你还需要引入一些await，方法是这样的：

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

C# Async ApiController 过早关闭 OutputStream

C# Async ApiController Closing OutputStream Prematurely

c#

asp.net-mvc

async-await

asp.net-web-api

asp.net-web-api2