C＃异步方法用法

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

Answer 1

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;
           ...

      }