Question

I’m having trouble invoking an event from a secondary thread in the main thread. The event handler is not executed on main thread. Can anyone give me some pointers on what I’m doing wrong.

Thanks

namespace ThreadSyncExample
{
  class Program
  {
    static void Main(string[] args)
    {
      Console.WriteLine("MainThread: " + System.Threading.Thread.CurrentThread.ManagedThreadId);

      Execute execThe = new Execute();
      execThe.FinishedThread += (src, arg) =>
      {
        //This shoould be executed on MainThread right?
        Console.WriteLine("Thread Id: " + System.Threading.Thread.CurrentThread.ManagedThreadId);
      };

      execThe.Run();
      Console.ReadKey();
    }

  }


  class Execute
  {
    public void Run()
    {
      Thread exec = new Thread(() =>
      {
        Console.WriteLine("Worker Thread : " + System.Threading.Thread.CurrentThread.ManagedThreadId);
        OnFinishedThread();
      });

      exec.Start();
    }

    public event EventHandler FinishedThread;
    protected virtual void OnFinishedThread()
    {
      if (null != FinishedThread)
      {
        EventArgs args = new EventArgs();
        FinishedThread(this, EventArgs.Empty);
      }
    }
  }
}

Answer 1

C# events are basically just an easy-to-use collection of delegates and “firing” an event just causes the runtime to loop through all of the delegates and fire them one at a time.

So your OnFinishedThread event handler is getting called on the Worker thread.

If you want your event on the main thread, you have to Invoke() it.

EDIT :

It appears that you don’t have access to forms, or WPF (so you don’t have access to Invoke() either)

So you have to manually marshall the call to the main thread by thread synchronization process. It’s generally a pain.

Probably the easiest solution would be to simply use a BackgroundWorker because this way you no longer need to manualy marshal the calls to the main thread.

var worker = new BackgroundWorker();
worker.DoWork += (sender, e) =>
{
    // call the XYZ function
    e.Result = XYZ();
};
worker.RunWorkerCompleted += (sender, e) =>
{
    // use the result of the XYZ function:
    var result = e.Result;
    // Here you can safely manipulate the GUI controls
};
worker.RunWorkerAsync();