Question

AFAIK, this event is promised to be called in the creator thread (BackgroundWorker RunWorkerCompleted Event); and in the most cases, it does as promised. However, sometimes RunWorkerCompleted is called not in the thread that created BackgroundWorker object.

Update: Code is like this:

Trace.WriteLine(Thread.CurrentThread.ManagedThreadId);
var worker = new BackgroundWorker();
worker.DoWork += (s, args) => Trace.WriteLine(Thread.CurrentThread.ManagedThreadId);
worker.RunWorkerCompleted += (s, args) => Trace.WriteLine(Thread.CurrentThread.ManagedThreadId);
worker.RunWorkerAsync();

Output is 1 7 7 (correct output must be 1 7 1)
Any ideas?

Answer 1

BackgroundWorker is designed for GUI applications.

I’m guessing you’re doing this in a console application or some other type of application other than Winforms or WPF.

BackgroundWorker uses the synchronization model provided by SynchronizationContext.Current to dispatch events. In a GUI application, SynchronizationContext.Current is initialized with a WindowsFormsSynchronizationContext, which provides synchronization by Invoking on the UI thread.

But in a non-GUI application, SyncronizationContext.Current is just a SynchronizationContext object, which (from MSDN):

is a base class that provides a free-threaded context with no synchronization.

In other words it simply dispatches via the threadpool, so you’ll usually get a different thread each time.

If you run your code in a winforms application, it will work as expected.