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Comparing BackgroundWorker and async/await

BackgroundWorker is meant to model a single task that you’d want to perform in the background, on a thread pool thread. async/await is a syntax for asynchronously awaiting on asynchronous operations. Those operations may or may not use a thread pool thread or even use any other thread. So, they’re apples and oranges.

For example, you can do something like the following with await:

using (WebResponse response = await webReq.GetResponseAsync())
{
    using (Stream responseStream = response.GetResponseStream())
    {
        int bytesRead = await responseStream.ReadAsync(buffer, 0, buffer.Length);
    }
}

But, you’d likely never model that in a background worker, you’d likely do something like this in .NET 4.0 (prior to await):

webReq.BeginGetResponse(ar =>
{
    WebResponse response = webReq.EndGetResponse(ar);
    Stream responseStream = response.GetResponseStream();
    responseStream.BeginRead(buffer, 0, buffer.Length, ar2 =>
    {
        int bytesRead = responseStream.EndRead(ar2);
        responseStream.Dispose();
        ((IDisposable) response).Dispose();
    }, null);
}, null);

Notice the disjointness of the disposal compared between the two syntaxes and how you can’t use using without async/await.

But, you wouldn’t do something like that with BackgroundWorker. BackgroundWorker is usually for modeling a single long-running operation that you don’t want to impact the UI responsiveness. For example:

worker.DoWork += (sender, e) =>
                    {
                    int i = 0;
                    // simulate lengthy operation
                    Stopwatch sw = Stopwatch.StartNew();
                    while (sw.Elapsed.TotalSeconds < 1)
                        ++i;
                    };
worker.RunWorkerCompleted += (sender, eventArgs) =>
                                {
                                    // TODO: do something on the UI thread, like
                                    // update status or display "result"
                                };
worker.RunWorkerAsync();

There’s really nothing there you can use async/await with, BackgroundWorker is creating the thread for you.

Now, you could use TPL instead:

var synchronizationContext = TaskScheduler.FromCurrentSynchronizationContext();
Task.Factory.StartNew(() =>
                      {
                        int i = 0;
                        // simulate lengthy operation
                        Stopwatch sw = Stopwatch.StartNew();
                        while (sw.Elapsed.TotalSeconds < 1)
                            ++i;
                      }).ContinueWith(t=>
                                      {
                                        // TODO: do something on the UI thread, like
                                        // update status or display "result"
                                      }, synchronizationContext);

In which case the TaskScheduler is creating the thread for you (assuming the default TaskScheduler), and could use await as follows:

await Task.Factory.StartNew(() =>
                  {
                    int i = 0;
                    // simulate lengthy operation
                    Stopwatch sw = Stopwatch.StartNew();
                    while (sw.Elapsed.TotalSeconds < 1)
                        ++i;
                  });
// TODO: do something on the UI thread, like
// update status or display "result"

In my opinion, a major comparison is whether you’re reporting progress or not. For example, you might have a BackgroundWorker like this:

BackgroundWorker worker = new BackgroundWorker();
worker.WorkerReportsProgress = true;
worker.ProgressChanged += (sender, eventArgs) =>
                            {
                            // TODO: something with progress, like update progress bar

                            };
worker.DoWork += (sender, e) =>
                 {
                    int i = 0;
                    // simulate lengthy operation
                    Stopwatch sw = Stopwatch.StartNew();
                    while (sw.Elapsed.TotalSeconds < 1)
                    {
                        if ((sw.Elapsed.TotalMilliseconds%100) == 0)
                            ((BackgroundWorker)sender).ReportProgress((int) (1000 / sw.ElapsedMilliseconds));
                        ++i;
                    }
                 };
worker.RunWorkerCompleted += (sender, eventArgs) =>
                                {
                                    // do something on the UI thread, like
                                    // update status or display "result"
                                };
worker.RunWorkerAsync();

But, you wouldn’t deal with some of this because you’d drag-and-drop the background worker component on to the design surface of a form–something you can’t do with async/await and Task… i.e. you won’t manually create the object, set the properties and set the event handlers. you’d only fill in the body of the DoWork, RunWorkerCompleted, and ProgressChanged event handlers.

If you “converted” that to async/await, you’d do something like:

     var progress = new Progress<int>();

     progress.ProgressChanged += ( s, e ) =>
        {
           // TODO: do something with e.ProgressPercentage
           // like update progress bar
        };

await Task.Factory.StartNew(() =>
                  {
                    int i = 0;
                    // simulate lengthy operation
                    Stopwatch sw = Stopwatch.StartNew();
                    while (sw.Elapsed.TotalSeconds < 1)
                    {
                        if ((sw.Elapsed.TotalMilliseconds%100) == 0)
                        {
                            progress.OnReport((int) (1000 / sw.ElapsedMilliseconds))
                        }
                        ++i;
                    }
                  });
// TODO: do something on the UI thread, like
// update status or display "result"

Without the ability to drag a component on to a Designer surface, it’s really up to the reader to decide which is “better”. But, that, to me, is the comparison between await and BackgroundWorker, not whether you can await built-in methods like Stream.ReadAsync. e.g. if you were using BackgroundWorker as intended, it could be hard to convert to use await.

with :