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Asked: 2026-06-13T00:37:02+00:00

I am working with Web Api to create a way to upload files via

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I am working with Web Api to create a way to upload files via web api. I have found several blog posts on how to accomplish this, and the code is all very similar with a key commonality being the Request.Content.ReadAsMultipartAsync() call. The problem I have is the first upload works fine, but then IIS gets into a faulted state where subsequent uploads fail. The first 32Kb comes in, but then it quits. Debugging shows only a null reference exception that occurs somewhere in the ASP.NET framework.

Here is the ApiController definition I have…

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"));
        }
    }
}

Also, here is the page I am posting from…

<!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>

The above code can be downloaded in a default WebApi solution from https://github.com/JohnLivermore/FileUploadTest. Run and navigate to http://localhost:{port}/FormPost.html. The first upload succeeds (uploads to App_Data), but subsequent uploads only upload the first 32 Kb and then fail.

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1 Answer

  1. Editorial Team

    You shouldn’t use a void method.

    Void and async don’t play well together for a number of reasons.

       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"));
   }
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