Question on how to show a modal dialog synchronously

In WPF, the Window.ShowDialog(); method blocks the thread and continues when the dialog is closed (the same way MessageBox.Show(); works in WinForms). In Avalonia, the docs indicate that Window.ShowDialog() executes asynchronously, meaning the thread can continue before the dialog is closed.

Maybe this is a matter of me not being good at async vs sync coding but how would I produce a similar result to WPF/WinForms but using the way Avalonia has been architected? I want to call ShowDialog from a non-async method and have execution pause until the dialog is closed.

I’ve tried the following:

Task.WhenAll(dialog.ShowDialog(parent));, dialog.ShowDialog(parent).Result;, and dialog.ShowDialog(parent).Wait(); block the dialog thread and prevents the dialog from loading.

Task.Run(() => ShowDialog()).GetAwaiter().GetResult();
private async Task ShowDialog()
{
    Dialog dialog = new Dialog();
    await dialog.ShowDialog(parent);
}

throws an exception of System.InvalidOperationException: ‘Call from invalid thread’

What is the proper way to do this?

[jp2masa]

A workaround (adapted from https://github.com/AvaloniaUI/Avalonia/issues/857#issuecomment-272927078) is this:

https://github.com/jp2masa/dotnet-properties/blob/4584f2f4245d999eb270ece5ec013f9f97c0f8bf/src/dotnet-properties/Services/DialogService.cs#L28-L32

It will block until the dialog closes.

[sn4k3]

@derekantrican if that works fine for you and you use it for multiple windows, consider making extension to ease your code. Something like:

public static void ShowDialogSync(this Window window, Window parent = null)
        {
            if (parent is null) parent = window;
            using (var source = new CancellationTokenSource())
            {
                window.ShowDialog(parent).ContinueWith(t => source.Cancel(), TaskScheduler.FromCurrentSynchronizationContext());
                Dispatcher.UIThread.MainLoop(source.Token);
            }
        }

        public static T ShowDialogSync<T>(this Window window, Window parent = null)
        {
            if (parent is null) parent = window;
            using (var source = new CancellationTokenSource())
            {
                var task = window.ShowDialog<T>(parent);
                task.ContinueWith(t => source.Cancel(), TaskScheduler.FromCurrentSynchronizationContext());
                Dispatcher.UIThread.MainLoop(source.Token);
                return task.Result;
            }

            return default(T);
        }