C# Async method usage

Question

I am trying to use Aync method. It's working in a way. But it's not working in some other context.

Working example

dropBox.DownloadFileAsync(csvEntry.Path)
                    .ContinueWith(task =>
                    {
                        // Save file to "C:\Spring Social.txt"
                        using (FileStream fileStream = new FileStream(tempCsvPath, FileMode.Create))
                        {
                            fileStream.Write(task.Result.Content, 0, task.Result.Content.Length);
                        }
                    });

Instead of saving the file, I am trying to return the byte array in the following way. But it's not working. It is returning null.

 byte[] returnArray = null;
                dropbox.DownloadFileAsync(filePath)
                        .ContinueWith(task =>
                        {
                            returnArray = new byte[task.Result.Content.Length];
                            task.Result.Content.CopyTo(returnArray, 0);
                        });
                return returnArray;

Can somebody correct me?

Thanks

Surely DownloadFileAsync is asynchronous, and so therefore you should not expect it to have completed by the time you hit the next statement (your "return" statement)? — Gary McGill
– Gary McGill, Commented Dec 12, 2012 at 23:56

thumbmunkeys · Accepted Answer · 2012-12-13 01:03:58Z

4

In this code:

            byte[] returnArray = null;
            dropbox.DownloadFileAsync(filePath)
                    .ContinueWith(task =>
                    {
                        returnArray = new byte[task.Result.Content.Length];
                        task.Result.Content.CopyTo(returnArray, 0);
                    });
            return returnArray;

DownloadFileAsync() is executed on a thread. When the ContinueWith() is executed your function that calls DowloadFileAsync() already has returned.

You would need to do something like this:

     Task<T> Download(string filePath)
     {             
            return dropbox.DownloadFileAsync(filePath)
                    .ContinueWith(task =>
                    {
                        returnArray = new byte[task.Result.Content.Length];
                        task.Result.Content.CopyTo(returnArray, 0);

                        return returnArray;
                    });
      }

Call it like this:

     var task = Download("myfile");

     task.ContinueWith(t => 
      { 
           var returnArray = t.Result;
           ...

      }

edited Dec 13, 2012 at 1:03

answered Dec 12, 2012 at 23:57

thumbmunkeys

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6 Comments

Naresh Over a year ago

Thanks for the reply. I have changed my code as you mentioned and I am relying on task.Result where task is returned by Download method. Is it the right way of doing it?

svick Over a year ago

It's not “executed on a background thread”. A big point of using async is that it doesn't block any thread when doing IO.

thumbmunkeys Over a year ago

it's still a thread where the async code is executed, or not?

igrimpe Over a year ago

It's not “executed on a background thread -> it IS executed in the background, but when you say something = t.result; you BLOCK the current thread until the task is done. If you dont want that, use "await" (if you use VS2012 and target FW 4.x)

thumbmunkeys Over a year ago

@igrimpe but the something = t.result is only executed in the continuation, so the thread (UI thread in this case I guess), where Download() is called, is not blocked, or not?

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