等待PostAsync在mscorlib中抛出NullReferenceException

Question

我需要从一个Mvc4应用程序发送文件到另一个(另一个是Mvc4,WebApi应用程序).为了发送我使用HttpClient的PostAsync方法.以下是执行发送的代码:

public class HomeController : Controller
{
    public async Task<ActionResult> Index()
    {
        var result =
            await Upload("http://localhost/target/api/test/post", "Test", System.IO.File.Open(@"C:\SomeFile", FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite));

        return View(result);
    }

    public async Task<String> Upload(String url, string filename, Stream stream)
    {
        using (var client = new HttpClient())
        {
            var formData = new MultipartFormDataContent();

            var fileContent = new StreamContent(stream);

            var header = new ContentDispositionHeaderValue("attachment") { FileName = filename };

            fileContent.Headers.ContentDisposition = header;

            formData.Add(fileContent);

            var result = await client.PostAsync(url, formData); // Use your url here

            return "123";
        }
    }
}

为了接收,我使用官方web api教程中的一个示例,这是执行它的代码:

public class TestController : ApiController
{
    // POST api/values
    public Task<HttpResponseMessage> Post()
    {
        if (!Request.Content.IsMimeMultipartContent())
        {
            throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
        }

        string root = HttpContext.Current.Server.MapPath("~/App_Data");


        var provider = new MultipartFormDataStreamProvider(root);


        // Read the form data and return an async task.
        var task = Request.Content.ReadAsMultipartAsync(provider).ContinueWith(t =>
        {
            if (t.IsFaulted || t.IsCanceled)
            {
                return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, t.Exception);
            }

            // This illustrates how to get the file names.
            foreach (MultipartFileData file in provider.FileData)
            {
                Trace.WriteLine(file.Headers.ContentDisposition.FileName);
                Trace.WriteLine("Server file path: " + file.LocalFileName);

            }

            return new HttpResponseMessage
            {
                Content = new StringContent("File uploaded.")
            };

            //var response = Request.CreateResponse(HttpStatusCode.OK);

            //return response;
        });

        return task;
    }
}

问题:

接收方成功获取文件,但是当它使用以下代码进行响应时:

return new HttpResponseMessage
{
    Content = new StringContent("File uploaded.")
};

发送方是在.Net的mscorlib内部深处的某个地方打破.即使我await用try/catch 包装调用,也不会处理异常.

我想保留异步实现,不想使用同步,这可能吗？为什么会出现问题？有什么想法吗？

Answer 1

仔细检查您的教程代码.我假设你在谈论这个页面,在这种情况下你应该使用.NET 4.5版本的代码(with await),而不是旧的.NET 4.0示例.

ASP.NET内在函数(HttpContext,请求,响应等)只能从请求上下文中访问.您的代码正在使用ContinueWith而未指定a TaskScheduler,在这种情况下将导致lambda 在请求上下文之外的线程池上运行.

在async代码中,你根本不应该使用ContinueWith(除非你真的需要它); await改为使用:

// POST api/values
public async Task<HttpResponseMessage> Post()
{
    if (!Request.Content.IsMimeMultipartContent())
    {
        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    }

    string root = HttpContext.Current.Server.MapPath("~/App_Data");
    var provider = new MultipartFormDataStreamProvider(root);

    try
    {
        // Read the form data.
        await Request.Content.ReadAsMultipartAsync(provider);

        // This illustrates how to get the file names.
        foreach (MultipartFileData file in provider.FileData)
        {
            Trace.WriteLine(file.Headers.ContentDisposition.FileName);
            Trace.WriteLine("Server file path: " + file.LocalFileName);
        }

        return new HttpResponseMessage
        {
            Content = new StringContent("File uploaded.")
        };
    }
    catch (Exception ex)
    {
        return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, ex);
    }
}