Multiple calls to MessageDialog cause crash under Windows Phone 8.1

Question

I develop an Universal App that uses MVVM-Light. I call WebServices from the ViewModels, and I throw the exceptions encountered by the calls at the WebServices to the ViewModels: TimeOut, Wrong URL, Server Exception, ...

I have created a class "ExceptionsMsgHelper.cs" which centralizes the messages displayed for each of these exceptions through MessageDialog.

My HomePage is based on a Pivot that containing several datas: some WebServices are called asynchronously. I so meet a crash if I show an Exception in a MessageDialog through the class "ExceptionsMsgHelper.cs", whereas a previous Exception is also showed in another MessageDialog.

Here is a part of my original class:

    public class ExceptionsMsgHelper
    {
        public async static void MsgboxWebserviceErrors(WebServiceErrorsException wseE, string errors)
        {
            Windows.UI.Popups.MessageDialog msgbox =
            new Windows.UI.Popups.MessageDialog("The Websercice '" + wseE.WebService + "' has returned errors : \n" + errors,
                "Unexpected data");        
            await msgbox.ShowAsync();
        }
    }

=> If I call twice the "msgbox.ShowAsync()", I get the "System.UnauthorizedAccessException" Exception: with message "Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))"

I've so looked for solutions in order to fix it:

• use a "Dispatcter", like it is recommended here (WinRT - MessageDialog.ShowAsync will throw UnauthorizedAccessException in my custom class)

The code is:

    public class ExceptionsMsgHelper
    {
        public async static void MsgboxWebserviceErrors(WebServiceErrorsException wseE, string errors)
        {
            Windows.UI.Popups.MessageDialog msgbox =
            new Windows.UI.Popups.MessageDialog("The Websercice '" + wseE.WebService + "' has returned errors : \n" + errors,
                "Unexpected data");        
            CoreDispatcher dispatcher = CoreWindow.GetForCurrentThread().Dispatcher;
            dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
            {
                await msgbox.ShowAsync();
            });
        }
    }

=> But I always meet the same exception.

• use a "IAsyncOperation" command to close the previous MessageDialog, like recommended here (MessageDialog ShowAsync throws accessdenied exception on second dialog)

With this code:

    public class ExceptionsMsgHelper
    {
        private static IAsyncOperation<IUICommand> messageDialogCommand = null;
        public async static Task<bool> ShowDialog(MessageDialog dlg)
        {

            // Close the previous one out
            if (messageDialogCommand != null)
            {
                messageDialogCommand.Cancel();
                messageDialogCommand = null;
            }

            messageDialogCommand = dlg.ShowAsync();
            await messageDialogCommand;
            return true;
        }

        public async static void MsgboxWebserviceErrors(WebServiceErrorsException wseE, string errors)
        {
            Windows.UI.Popups.MessageDialog msgbox =
            new Windows.UI.Popups.MessageDialog("The Websercice '" + wseE.WebService + "' has returned errors : \n" + errors,
                "Unexpected data");        
            CoreDispatcher dispatcher = CoreWindow.GetForCurrentThread().Dispatcher;
            dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
            {
                await ShowDialog(msgbox);
            });
        }           
    }

=> But in this case too, I always get the same exception.

• use an extension to queue up messagedialogs, like describing here (Multiple MessageDialog app crash)

The code is now:

    public class ExceptionsMsgHelper
    {
        public async static void MsgboxWebserviceErrors(WebServiceErrorsException wseE, string errors)
        {
            Windows.UI.Popups.MessageDialog msgbox =
            new Windows.UI.Popups.MessageDialog("The Websercice '" + wseE.WebService + "' has returned errors : \n" + errors,
                "Unexpected data");   
            await MessageDialogExtensions.ShowAsyncQueue(msgbox);
        }
    }

    public static class MessageDialogExtensions
    {
        private static TaskCompletionSource<MessageDialog> _currentDialogShowRequest;
        public static async Task<IUICommand> ShowAsyncQueue(this MessageDialog dialog)
        {
            if (!Window.Current.Dispatcher.HasThreadAccess)
            {
                throw new InvalidOperationException("This method can only be invoked from UI thread.");
            }

            while (_currentDialogShowRequest != null)
            {
                await _currentDialogShowRequest.Task;
            }

            var request = _currentDialogShowRequest = new TaskCompletionSource<MessageDialog>();
            var result = await dialog.ShowAsync();
            _currentDialogShowRequest = null;
            request.SetResult(dialog);

            return result;
        }
    private static IAsyncOperation<IUICommand> messageDialogCommand = null;
    public async static Task<bool> ShowDialog(this MessageDialog dlg)
    {

        // Close the previous one out
        if (messageDialogCommand != null)
        {
            messageDialogCommand.Cancel();
            messageDialogCommand = null;
        }

        messageDialogCommand = dlg.ShowAsync();
        await messageDialogCommand;
        return true;
    }

    #endregion
    }

=> And this works for me.

But like says it's author, it's probably not the best solution:

Close existing dialog when you need to open a new one. This is the simplest option and possibly the best, although you risk cancelling a dialog that might be somehow important depending on what your dialogs are about. Queue up dialogs so the old ones don't get dismissed, but the new ones show up after the old ones were dismissed. This one will make sure all dialogs are closed by the user, but that could be a problem if your app can somehow start showing hundreds of dialogs. Only open a new one if there isn't one already displayed. Now this risks that a newer message is not shown, which sounds more problematic than the first option.

=> I would like to understand why I can't apply one the 2 first solutions that seems to be more adapted

Answer 1

Ofcourse you can't show 2 or more message dialog at the same time (windows phone limits). Moreover MesssageDialog on Windows Phone 8.1 has probably bug and can't be closed.

If closing previous dialog will be solution for you, try to use ContentDialog instead MessageDialog. Check my answer in this topic: Closing MessageDialog programatically in WP 8.1 RT

I think it solve your problem.