MessageDialog ShowAsync throws accessdenied exception on second dialog

问题

I am trying to implement try again/cancel dialog box in windows 8. The dialog box shows fine the first time, but on clicking try again and failing again, I get a access denied exception on calling ShowAsync. I don\'t know why, but its strange sometimes the code works fine and I don\'t get the exception when I put breakpoints. really clueless here

here is the code.

    async void DismissedEventHandler(SplashScreen sender, object e)
    {
        dismissed = true;
        loadFeeds();
    }
    private async void loadFeeds()
    {
        await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
        {
            try
            {
                RSSDataSource rssDataSource = (RSSDataSource)App.Current.Resources[\"RSSDataSource\"];
                if (rssDataSource != null)
                {
                    await rssDataSource.DownloadFeeds();
                    await rssDataSource.GetFeedsAsync();
                }

                AdDataSource ads = (AdDataSource)App.Current.Resources[\"AdDataSource\"];

                if (ads != null)
                {
                    await ads.DownloadAds();
                }
                rootFrame.Navigate(typeof(HomePageView));

                Window.Current.Content = rootFrame;
            }
            catch
            {
                ShowError();
            }

        });
    }
    async void ShowError()
    {
        // There was likely a problem initializing
        MessageDialog msg = new MessageDialog(CONNECTION_ERROR_MESSAGE, CONNECTION_ERROR_TITLE);

        // Add buttons and set their command handlers
        msg.Commands.Add(new UICommand(COMMAND_LABEL_RETRY, new UICommandInvokedHandler(this.CommandInvokedHandler)));
        msg.Commands.Add(new UICommand(COMMAND_LABEL_CLOSE, new UICommandInvokedHandler(this.CommandInvokedHandler)));
        // Set the command to be invoked when a user presses \'ESC\'
        msg.CancelCommandIndex = 0;

        await msg.ShowAsync();
    }

    /// <summary>
    /// Callback function for the invocation of the dialog commands
    /// </summary>
    /// <param name=\"command\">The command that was invoked</param>
    private void CommandInvokedHandler(IUICommand command)
    {
        string buttonLabel = command.Label;
        if (buttonLabel.Equals(COMMAND_LABEL_RETRY))
        {
            loadFeeds();
        }
        else
        {
            // Close app
            Application.Current.Exit();
        }
    }

回答1:

Okay I found a quick solution,

define a IAsyncOperation class varialble

IAsyncOperation<IUICommand> asyncCommand = null;

and set it to the ShowAsync method of MessageDialog

asyncCommand = msg.ShowAsync();

In the command handler for retry/try again check if asyncCommand is not null and cancel the last operation if necessary

if(asyncCommand != null)
{
   asyncCommand.Cancel();
}

Please let me if there is a better approach to this.

回答2:

I am late to the party, but here's a way where you can always await the result of the dialog box, as well as not need to worry about calling too many in a row:

First define a static variable and method in your application:

 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;
 }

Now, you can pass in any dialog box and always await execution. This is why this returns a bool rather than void. You won't have to worry about collisions between multiples. Why not make this method accept a string? Because of title, and Yes/No command handlers that you may assign into the specific dialog box you are using.

Invoke such as:

await App.ShowDialog(new MessageDialog("FOO!"));

or

var dlg = new MessageDialog("FOO?", "BAR?");
dlg.Commands.Add(new UICommand("Yes", new UICommandInvokedHandler(YesHandler)));
dlg.Commands.Add(new UICommand("No", new UICommandInvokedHandler(NoHandler)));
await App.ShowDialog(dlg);

回答3:

There is an answer for this on the MSDN forums that might help you here.

http://social.msdn.microsoft.com/Forums/en-US/winappswithhtml5/thread/c2f5ed68-aac7-42d3-bd59-dbf2673dd89b

I'm having a similar problem but my showAsync calls are in separate functions on separate threads so I can't drop a done() in there I don't think...

回答4:

I was facing this same issue some days ago, and i solve it awaiting the ShowAsync and then making the recursive call that open the MessageDialog again.

public async void ShowDlg(){
    Action cmdAction = null;
    var msgDlg = new MessageDialog("Content.", "Title");
    msgDlg.Commands.Add(new UICommand("Retry", (x) => {
    cmdAction = () => ShowDlg();
    }));
    msgDlg.Commands.Add(new UICommand("Cancel", (x) => {
    cmdAction = () => <Action associated with the cancel button>;
    }));
    msgDlg.DefaultCommandIndex = 0;
    msgDlg.CancelCommandIndex = 1;

    await msgDlg.ShowAsync();
    cmdAction.Invoke();
}

Hope this help!

回答5:

Another solution:

private bool _messageShowing = false;

// ...

if (!_messageShowing)
{
    _messageShowing = true;
    var messageDialog = new MessageDialog("Example");

    // ... "messageDialog" initialization

    Task<IUICommand> showMessageTask =  messageDialog.ShowAsync().AsTask();
    await showMessageTask.ContinueWith((showAsyncResult) =>
        {
            _messageShowing = false;
        });
}

来源：https://stackoverflow.com/questions/12722490/messagedialog-showasync-throws-accessdenied-exception-on-second-dialog