Question

What is the proper technique to have ThreadA signal ThreadB of some event, without having ThreadB sit blocked waiting for an event to happen?

i have a background thread that will be filling a shared List<T>. i’m trying to find a way to asynchronously signal the ‘main’ thread that there is data available to be picked up.

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i considered setting an event with an EventWaitHandle object, but i can’t have my main thread sitting at an Event.WaitOne().

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i considered having a delegate callback, but a) i don’t want the main thread doing work in the delegate: the thread needs to get back to work adding more stuff – i don’t want it waiting while the delegate executes, and b) the delegate needs to be marshalled onto the main thread, but i’m not running a UI, i have no Control to .Invoke the delegate against.

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i considered have a delegate callback that simply starts a zero interval System.Windows.Forms.Timer (with thread access to the timer synchronized). This way the thread only needs to be stuck as it calls

Timer.Enabled = true;

but that seems like a hack.

In the olden days my object would have created a hidden window and had the thread post messages to that hidden windows’ HWND. i considered creating a hidden control, but i gather that you cannot .Invoke on a control with no handle created. Plus, i have no UI: my object could have been created on a web-server, service, or console, i don’t want there to be a graphical control appearing – nor do i want to compile a dependency on System.Windows.Forms.

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i considered having my object expose an ISynchronizeInvoke interface, but then i would need to implement .Invoke(), and that’s my problem.

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What is the proper technique to have thread A signal thread B of some event, without having thread B sit blocked waiting for an event to happen?

Answer 1

Here’s a code sample for the System.ComponentModel.BackgroundWorker class.

    private static BackgroundWorker worker = new BackgroundWorker();     static void Main(string[] args)     {         worker.DoWork += worker_DoWork;         worker.RunWorkerCompleted += worker_RunWorkerCompleted;         worker.ProgressChanged += worker_ProgressChanged;         worker.WorkerReportsProgress = true;          Console.WriteLine('Starting application.');         worker.RunWorkerAsync();          Console.ReadKey();     }      static void worker_ProgressChanged(object sender, ProgressChangedEventArgs e)     {         Console.WriteLine('Progress.');     }      static void worker_DoWork(object sender, DoWorkEventArgs e)     {         Console.WriteLine('Starting doing some work now.');          for (int i = 0; i < 5; i++)         {             Thread.Sleep(1000);             worker.ReportProgress(i);         }     }      static void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)     {         Console.WriteLine('Done now.');     }