c# Win Store App: Thread error

[James Rose]

Hello gang,

 

I'm having an interesting error and looking at the what/how others are dealing with it is not helping.

 

I am using some code from MS so I can display visualizations within my app (code found here:  http://www.getcodesamples.com/src/5D67423E )

 

 

The interesting thing is I am getting the following error: The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

 

Here is the code to start the player, this works fine:

private MMAudioPlayer.Player BediaAudioPlayer

async private void PlayAudio(MenuItem BediaMenu)
            {
                try
                {
                    if (BediaAudioPlayer == null)
                    {
                        BediaAudioPlayer = new MMAudioPlayer.Player();
                    }

                    if (BediaMenu.MenuFile != null)
                    {
                        string sPath = BediaMenu.MenuFile.Path;
                        StorageFile AudioFile = await StorageFile.GetFileFromPathAsync(sPath);
                        IRandomAccessStream AudioStream = await AudioFile.OpenAsync(Windows.Storage.FileAccessMode.Read);
                        BediaAudioPlayer.SetAudioData(AudioStream);
                        BediaAudioPlayer.Vol = Convert.ToSingle(this.MediaVolume);
                    }
                }
                catch (Exception ex)
                {
                    LogException(ex);
                }
            }

However, when the audio completes in this code I get that error

 private void BediaAudioPlayer_OnEndOfStream(MMAudioPlayer.EndOfStreamReason paramaters)
            {
                try
                {
                    if (paramaters == MMAudioPlayer.EndOfStreamReason.ok)
                    {
                        mMainPage.BediaTitleBar.MenuTitle = "This is a test";    //error is here.
                    }
                }
                catch (Exception ex)
                {
                    LogException(ex);
                }
            }

mMainPage is the application's MainPage and all the code listed above is from my class BediaEngine.  I can call functions btw the two with no issues, with the exception of this one.  What am I doing wrong?

public MainPage()
        {
            this.InitializeComponent();
                        
            objBediaEngine = new BediaEngine(this); //The plan is that almost all of the work is done within this engine, only minor interfaces to the UI
}

[Eric Veteran]

Wrap your code with where you indicated the error is with that and it should work. :)

The EndOfStream event is being called by the main thread but the event handler code is actually called by a separate one (I think.)

Async methods run on their own thread so the event is essentially fire-and-forget so you have to use the dispatcher to send your event code to the original thread.

[Eric Veteran]

Try using this.Dispatcher.Invoke()

You're trying to change a control that's owned by another thread.

Let me make a test case to make sure I give you the right info and I'll update this.

 

            this.Dispatcher.Invoke(delegate()
            {
                // control access goes here
            });

[James Rose]

Try using this.Dispatcher.Invoke()

You're trying to change a control that's owned by another thread.

 

 

Eric,

 

You're always around, so thanks for that.

 

I got that the msg is saying that this is on another thread...  what I don't understand is how this is on another thread and how do I specifically resolve this?    I see you're statement on Dispatch, but I really don't understand which code to affect.

 

Yea, I'm sure I look stupid right now...  I know. (at least I will admit it)

 

EDIT: What I am trying to say is I'm trying to have more of an understanding than simply; "fix this for me"

[Lant]

GUI objects can only be modified from the thread that they were created in. So when you attempted to change it on a different thread an exception was thrown.
By sending it to the dispatcher, it makes sure that the GUI object is modified on the GUI thread.

[James Rose]

Eric,

 

Thanks, that was VERY helpful.  Not completely right for a Windows Store app, but so close that I got there.

 

If you're ever in NYC, drink on me.

private CoreDispatcher BediaDispatcher;


public BediaEngine(MainPage mainPage)
{
    BediaDispatcher = Window.Current.Dispatcher;
}



  private async void BediaAudioPlayer_OnEndOfStream(MMAudioPlayer.EndOfStreamReason paramaters)
            {
                try
                {
                    if (paramaters == MMAudioPlayer.EndOfStreamReason.ok)
                    {
                        await BediaDispatcher.RunAsync(CoreDispatcherPriority.Normal, () => 
                            {
                                AudioTimer.Stop();
                                MediaSelect(1);
                            });
                    }
                }
                catch (Exception ex)
                {
                    LogException(ex);
                }
            }