Async Best Practices for C# and Visual Basic

The Discussion

• 

  Exception management for asynchronous tasks can be a little tricky.

  In the following example, I have implemented exception management. Suppose that server returns an error message using the header "Reason-Phrase"

  public byte[] getFile (String url,String fileName)
  {
    WebRequest request = WebRequest.Create(url);
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";
    using (Stream stream = request.GetRequestStream())
    {
      /* suppose that the name of requested file is sent in the
         body of POST. For REST services you can send
         parameters in the body of POST, "url-encoded"
         and separated by "&" */
      byte[] content = Encoding.UTF8.GetBytes("name=" + WebUtility.UrlEncode(fileName));
      stream.Write(content, 0, content.Length);
    }
    // the following call will not block
    Task<WebResponse> task = request.GetResponseAsync();
    // ...here you can do something while task complete...
    try
    {
      // waiting until task complete
      // note exception management for Wait()
      task.Wait();
    }
    catch (AggregateException e)
    {
      throw new Exception(((WebException)e.InnerException).Response.Headers["Reason-Phrase"]);
    }
    using (Stream stream = task.Result.GetResponseStream())
      using (MemoryStream m = new MemoryStream())
      {
        stream.CopyTo(m);
        return m.ToArray();
      }
    }
  }

• 

  Fun session.  Thank you Mads!  You're one of my favorite speakers.

• 

  Where is the video?
  Thanks!

• 

  Where is the video?

• 

  In order to improve User Experience in front-end Apps, is a good practice implement the callback pattern (methods which are invoked when asynchronous operations are completed). This pattern is also a little tricky.

  Here an example (suppose that server returns an error message using the header "Reason-Phrase");

  public void getFileAsync(String url,String fileName,Action<byte[]> callback)
  {
    WebRequest request = WebRequest.Create(url);
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";
    using (Stream stream = request.GetRequestStream())
    {
      /* suppose that the name of requested file is sent in the
         body of POST. For REST services you can send
         parameters in the body of POST, "url-encoded"
         and separated by "&" */
      byte[] content = Encoding.UTF8.GetBytes("name=" + WebUtility.UrlEncode(fileName));
      stream.Write(content, 0, content.Length);
    }
    // the following call will not block
    Task<WebResponse> task = request.GetResponseAsync();
    if (callback != null) task.ContinueWith((t, c) =>
    {
      try
      {
        using (Stream stream = t.Result.GetResponseStream())
        using (MemoryStream m = new MemoryStream())
        {
          stream.CopyTo(m);
          ((Action<byte[]>)c)(m.ToArray());
        }
      }
      catch (AggregateException e)
      {
        throw ((WebException)e.InnerException).Response != null ? 
          new Exception(((WebException)e.InnerException).Response.Headers["Reason-Phrase"]) :
          e.InnerException;
      }
    }, callback);
  }

  Finally, you only need invoke the following method (the callback function is inlined using a lambda expression):

  getFileAsync("my-url","file-name",(buffer) => {
      // buffer contains the received file
    });

  Note that method getFileAsync() not blocks. Also, note that AggregateException is used for exception management because the anonymous function inlined within ContinueWith() executes in a different context.