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Task. Continued with execution order

Apparently, I don't understand how to use the ContinueWith method. My goal is to complete the task, and when it is completed, return the message.

Here is my code:

    public string UploadFile()
    {
        if (Request.Content.IsMimeMultipartContent())
        {
            //Save file
            MultipartFormDataStreamProvider provider = new MultipartFormDataStreamProvider(HttpContext.Current.Server.MapPath("~/Files"));
            Task<IEnumerable<HttpContent>> task = Request.Content.ReadAsMultipartAsync(provider);

            string filename = "Not set";

            task.ContinueWith(o =>
            {
                //File name
                filename = provider.BodyPartFileNames.First().Value;
            }, TaskScheduler.FromCurrentSynchronizationContext()); 

            return filename;
        }
        else
        {
            return "Invalid.";
        }
    }

The variable "filename" always returns "Not set." It seems that the code inside the ContinueWith method is never called. (It gets called if I debug it line by line in VS.)

This method is called in my ASP.NET/Ajax POST API.

What am I doing wrong here?

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4 answers

If you use an asynchronous operation, the best option would be to make your operation asynchronous, otherwise you will lose the benefits of the asynchronous call that you make. Try rewriting your method as follows:

public Task<string> UploadFile()
{
    if (Request.Content.IsMimeMultipartContent())
    {
        //Save file
        MultipartFormDataStreamProvider provider = new MultipartFormDataStreamProvider(HttpContext.Current.Server.MapPath("~/Files"));
        Task<IEnumerable<HttpContent>> task = Request.Content.ReadAsMultipartAsync(provider);

        return task.ContinueWith<string>(contents =>
        {
            return provider.BodyPartFileNames.First().Value;
        }, TaskScheduler.FromCurrentSynchronizationContext()); 
    }
    else
    {
        // For returning non-async stuff, use a TaskCompletionSource to avoid thread switches
        TaskCompletionSource<string> tcs = new TaskCompletionSource<string>();
        tcs.SetResult("Invalid.");
        return tcs.Task;
    }
}
+7

, :

  • , .
  • , , , , , " ". ( fileName).

:

public string UploadFile()
{
    if (Request.Content.IsMimeMultipartContent())
    {
        //Save file
        MultipartFormDataStreamProvider provider = new MultipartFormDataStreamProvider(HttpContext.Current.Server.MapPath("~/Files"));
        Task<IEnumerable<HttpContent>> task = Request.Content.ReadAsMultipartAsync(provider);

        string filename = "Not set";

        var finalTask = task.ContinueWith(o =>
            {
                //File name
                filename = provider.BodyPartFileNames.First().Value;
            }, TaskScheduler.FromCurrentSynchronizationContext()); 

        task.Start();

        finalTask.Wait();

        return filename;
    }
    else
    {
        return "Invalid.";
    }
}

:

  • task.ContinueWith finalTask. , .
  • ( task.Start();)
  • (finalTask.Wait();)

, , , ( ), , , , .

, ( ):

public string UploadFile()
{
    if (Request.Content.IsMimeMultipartContent())
    {
        //Save file
        MultipartFormDataStreamProvider provider = new MultipartFormDataStreamProvider(HttpContext.Current.Server.MapPath("~/Files"));

        Request.Content.ReadAsMultipart(provider); // don't know if this is really valid.

        return provider.BodyPartFileNames.First().Value;
    }
    else
    {
        return "Invalid.";
    }
}

: ; , , .

+2

Task<T> , Task<string>.

+1

asynch. , Wait :

task.ContinueWith(o =>
        {
            //File name
            filename = provider.BodyPartFileNames.First().Value;
        ).Wait();

return filename;

Edit: Some asynch methods start a task immediately after it is created, while others ask you to explicitly run them. You should consult the documentation for everyone to be sure. In this case, the task starts automatically.

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