c# - Request.Content.ReadAsMultipartAsync never returns

Question

I have an API for a system written using the ASP.NET Web Api and am trying to extend it to allow images to be uploaded. I have done some googling and found how the recommended way to accept files using MultpartMemoryStreamProvider and some async methods but my await on the ReadAsMultipartAsync never returns.

Here is the code:

[HttpPost]
public async Task<HttpResponseMessage> LowResImage(int id)
{
    if (!Request.Content.IsMimeMultipartContent())
    {
        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    }

    var provider = new MultipartMemoryStreamProvider();

    try
    {
        await Request.Content.ReadAsMultipartAsync(provider);

        foreach (var item in provider.Contents)
        {
            if (item.Headers.ContentDisposition.FileName != null)
            {

            }
        }

        return Request.CreateResponse(HttpStatusCode.OK);
    }
    catch (System.Exception e)
    {
        return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, e);
    }
}

I can step through all the way to:

await Request.Content.ReadAsMultipartAsync(provider);

at which point it will never complete.

What is the reason why my await never returns?

Update

I am attempting to POST to this action using curl, the command is as follows:

C:\cURL>curl -i -F filedata=@C:\LowResExample.jpg http://localhost:8000/Api/Photos/89/LowResImage

I have also tried using the following html to POST to the action as well and the same thing happens:

<form method="POST" action="http://localhost:8000/Api/Photos/89/LowResImage" enctype="multipart/form-data">
    <input type="file" name="fileupload"/>
    <input type="submit" name="submit"/>
</form>

Answer 1

我在 .NET 4.0 中遇到了类似的问题（没有异步/等待）。使用调试器的线程堆栈，我可以看出 ReadAsMultipartAsync 正在将任务启动到同一个线程上，因此它会死锁。我做了这样的事情：

IEnumerable<HttpContent> parts = null;
Task.Factory
    .StartNew(() => parts = Request.Content.ReadAsMultipartAsync().Result.Contents,
        CancellationToken.None,
        TaskCreationOptions.LongRunning, // guarantees separate thread
        TaskScheduler.Default)
    .Wait();

TaskCreationOptions.LongRunning 参数对我来说很关键，因为没有它，调用将继续在同一个线程上启动任务。您可以尝试使用类似下面的伪代码来查看它在 C# 5.0 中是否适用于您：

await TaskEx.Run(async() => await Request.Content.ReadAsMultipartAsync(provider))

Answer 2

Answer 3

在 stackoverflow 上的另一个答案和关于 targetFramework 的博客文章的帮助下，我发现更新到 4.5 并在 web.config 中添加/更新以下内容可以解决此问题：

<system.web>
    <compilation debug="true" targetFramework="4.5"/>
</system.web>
<appSettings>
    <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />
</appSettings>

Answer 4

我有一个有效的 .Net MVC WebAPi 项目，它使用以下 Post 方法，似乎效果很好。它与您已经拥有的非常相似，因此这应该会有所帮助。

    [System.Web.Http.AcceptVerbs("Post")]
    [System.Web.Http.HttpPost]
    public Task<HttpResponseMessage> Post()
    {
        // Check if the request contains multipart/form-data.
        if (!Request.Content.IsMimeMultipartContent())
        {
            throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
        }
        string fileSaveLocation = @"c:\SaveYourFile\Here\XXX";
        CustomMultipartFormDataStreamProvider provider = new CustomMultipartFormDataStreamProvider(fileSaveLocation);
        Task<HttpResponseMessage> task = Request.Content.ReadAsMultipartAsync(provider).ContinueWith<HttpResponseMessage>(t =>
            {
                if (t.IsFaulted || t.IsCanceled)
                {
                    Request.CreateErrorResponse(HttpStatusCode.InternalServerError, t.Exception);
                }
                foreach (MultipartFileData file in provider.FileData)
                {
                    //Do Work Here
                }
                return Request.CreateResponse(HttpStatusCode.OK);
            }
        );
        return task;
    }

Answer 5

我有同样的。我的解决方案

public List<string> UploadFiles(HttpFileCollection fileCollection)
    {
        var uploadsDirectoryPath = HttpContext.Current.Server.MapPath("~/Uploads");
        if (!Directory.Exists(uploadsDirectoryPath))
            Directory.CreateDirectory(uploadsDirectoryPath);

        var filePaths = new List<string>();

        for (var index = 0; index < fileCollection.Count; index++)
        {
            var path = Path.Combine(uploadsDirectoryPath, Guid.NewGuid().ToString());
            fileCollection[index].SaveAs(path);
            filePaths.Add(path);
        }

        return filePaths;
    }

并调用

if (!Request.Content.IsMimeMultipartContent())
{
    throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
}

var filePaths = _formsService.UploadFiles(HttpContext.Current.Request.Files);