Question

今天的问题是，当使用WebApi 2和基于Async ApiController的Get方法时，返回文件的内容。当我将Get方法更改为同步时，它可以正常工作，但只要我将其转换回异步，它就会过早地关闭流。（Fiddler报告连接中止）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());
        }
    }

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----------

Async版本完全相同，只有：

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，如下所示：（streamToCopyTo是response.OutputStream中的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);
    }
}

我们试图从前到后采用异步/等待模式，所以希望有人知道为什么会失败？我也尝试过不调用Response.End（），Response.Flush（）和HttpContext.Current.ApplicationInstance.CompleteRequest（）。另外，在回答下面的问题/评论时，我在response.End（）上放置了一个断点，其结果是没有点击GetDownloadStream方法已经完成。也许OutputStream不是异步的？欢迎任何想法！感谢

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

非常感谢所有评论过的人，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如下所示：

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

我遗漏了我还在动态解密文件流，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

问题的根源实际上在于使用Response.End()。运行Async时，它将在完成文件内容流式传输之前执行Response.End()。使用同步版本时不会出现这种情况，因为在完成文件内容流式传输之后才会调用Response.End()。

Response.End()是一种特别糟糕的方式，表示你已完成处理，因为它会抛出TreadAbortException。相反，您应该使用HttpContext.Current.ApplicationInstance.CompleteRequest()

有关详情Response.End, Response.Close, and How Customer Feedback Helps Us Improve MSDN Documentation

，请参阅此文章

Answer 2

我们需要有一个完整的复制案例才能回答您的确切问题，但我认为您根本不需要async/await。我还认为你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;
}

此处没有明确的异步，因此该方法不是async。但是，如果您仍然需要引入一些await，则方法如下：

[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

2 个答案: