Question

我正在使用Web Api创建一种通过web api上传文件的方法。我发现了几篇关于如何实现这一目标的博客文章，代码非常类似于一个关键的共性，即Request.Content.ReadAsMultipartAsync（）调用。我遇到的问题是第一次上传工作正常，但随后IIS进入故障状态，后续上传失败。第一个32Kb进来，但随后退出。调试仅显示在ASP.NET框架中某处发生的空引用异常。

这是我有的ApiController定义......

public class FileUploadController : ApiController
{
    public void Post()
    {
        if (Request.Content.IsMimeMultipartContent())
        {
            var path = HttpContext.Current.Server.MapPath("~/App_Data");
            var provider = new MultipartFormDataStreamProvider(path);
            var task = Request.Content.ReadAsMultipartAsync(provider);
            task.ContinueWith(t =>
            {
                if (t.IsFaulted || t.IsCanceled)
                    throw new HttpResponseException(HttpStatusCode.InternalServerError);
            });
        }
        else
        {
            throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.NotAcceptable, "This request is not properly formatted"));
        }
    }
}

此外，这是我发布的页面......

<!doctype html>
<head>
    <title>File Upload Progress Demo #3</title>
</head>
<body>
    <h1>File Upload Progress Demo #3</h1>
    <code>&lt;input type="file" name="myfile[]"></code><br>
    <form action="/api/fileupload" method="post" enctype="multipart/form-data">
    <input type="file" name="myfile"><br>
    <input type="submit" value="Upload File to Server">
    </form>

    <div class="progress">
        <div class="bar"></div>
        <div class="percent">0%</div>
    </div>

    <div id="status"></div>
</body>

上述代码可以在https://github.com/JohnLivermore/FileUploadTest的默认WebApi解决方案中下载。运行并导航至http://localhost:{port}/FormPost.html。第一次上传成功（上传到App_Data），但后续上传仅上传前32 Kb然后失败。

Answer 1

代码的主要问题是退出方法而不等待所有异步任务完成。您可以将.Wait()用于此目的：

public class FileUploadController : ApiController
{
    public void Post()
    {
        if (Request.Content.IsMimeMultipartContent())
        {
            var path = HttpContext.Current.Server.MapPath("~/App_Data");
            var provider = new MultipartFormDataStreamProvider(path);
            var readAsMultipartTask = Request.Content.ReadAsMultipartAsync(provider);
            var continueWithTask = readAsMultipartTask.ContinueWith(t =>
            {
                if (t.IsFaulted || t.IsCanceled)
                throw new HttpResponseException(HttpStatusCode.InternalServerError);
            });
            continueWithTask.Wait();
        }
        else
        {
            throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.NotAcceptable, "This request is not properly formatted"));
        }
    }
}

这将使上传工作正常，但您应该知道您的方法违反了HTTP协议，因为您没有在正确上传的情况下发回任何响应。你的方法应该更像这样：

public class FileUploadController : ApiController
{
    public async Task<HttpResponseMessage> Post()
    {
        if (Request.Content.IsMimeMultipartContent())
        {
            var path = HttpContext.Current.Server.MapPath("~/App_Data");
            var provider = new MultipartFormDataStreamProvider(path);
            await Request.Content.ReadAsMultipartAsync(provider).ContinueWith(t =>
            {
                if (t.IsFaulted || t.IsCanceled)
                    throw new HttpResponseException(HttpStatusCode.InternalServerError);
            });
            //Here you should return a meaningful response
            return Request.CreateResponse(HttpStatusCode.OK); 
        }
        else
        {
            throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.NotAcceptable, "This request is not properly formatted"));
        }
    }
}

（感谢使用async / await异步任务的同步由框架处理）

Answer 2

您不应该使用void方法。

由于多种原因，虚空和异步不能很好地协同工作。

   public Task<HttpResponseMessage> Post()
   {
      var rootUrl = "c:/uploads";

      if (Request.Content.IsMimeMultipartContent())
      {
         var streamProvider = new MultipartFormDataStreamProvider(rootUrl);
         var task = Request.Content.ReadAsMultipartAsync(streamProvider).ContinueWith<HttpResponseMessage>(t =>
         {
            if (t.IsFaulted || t.IsCanceled)
               throw new HttpResponseException(HttpStatusCode.InternalServerError);

            //do stuff with files if you wish

            return new HttpResponseMessage(HttpStatusCode.OK);
         });
      return task;
      }

      throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.NotAcceptable, "This request is not properly formatted"));
   }

第二次上传时，通过Web Api上传的文件失败

2 个答案: