Call async method on UI thread

.net-4.7, async-await, c#

Solution

As with other similar cases where you need to introduce asynchrony to a legacy app without much refactoring, I'd recommend using a simple "Please wait..." modal dialog. The dialog initiates an async operation and closes itself when the operation has finished.

`Window.ShowDialog` is a synchronous API in the way it blocks the main UI and only returns to the caller when the modal dialog has been closed. However, it still runs a nested message loop and pumps messages. Thus, the asynchronous task continuation callbacks still get pumped and executed, as opposed to using a deadlock-prone `Task.Wait()`.

Here is a basic but complete WPF example, mocking up `_oidcClient.LoginAsync()` with `Task.Delay()` and executing it on the UI thread, refer to `WpfTaskExt.Execute` for the details.

Cancellation support is optional; if the actual `LoginAsync` can't be cancelled, the dialog is prevented from being closed prematurely.

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;

namespace WpfApp1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            var button = new Button() { Content = "Login", Width = 100, Height = 20 };
            button.Click += HandleLogin;
            this.Content = button;
        }

        // simulate _oidcClient.LoginAsync
        static async Task<bool> LoginAsync(CancellationToken token)
        {
            await Task.Delay(5000, token);
            return true;
        }

        void HandleLogin(object sender, RoutedEventArgs e)
        {
            try
            {
                var result = WpfTaskExt.Execute(
                    taskFunc: token => LoginAsync(token),
                    createDialog: () =>
                        new Window
                        {
                            Owner = this,
                            Width = 320,
                            Height = 200,
                            WindowStartupLocation = WindowStartupLocation.CenterOwner,
                            Content = new TextBox
                            {
                                Text = "Loggin in, please wait... ",
                                HorizontalContentAlignment = HorizontalAlignment.Center,
                                VerticalContentAlignment = VerticalAlignment.Center
                            },
                            WindowStyle = WindowStyle.ToolWindow
                        },
                    token: CancellationToken.None);

                MessageBox.Show($"Success: {result}");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }

    public static class WpfTaskExt
    {
        /// <summary>
        /// Execute an async func synchronously on a UI thread,
        /// on a modal dialog's nested message loop
        /// </summary>
        public static TResult Execute<TResult>(
            Func<CancellationToken, Task<TResult>> taskFunc,
            Func<Window> createDialog,
            CancellationToken token = default(CancellationToken))
        {
            var cts = CancellationTokenSource.CreateLinkedTokenSource(token);

            var dialog = createDialog();
            var canClose = false;
            Task<TResult> task = null;

            async Task<TResult> taskRunner()
            {
                try
                {
                    return await taskFunc(cts.Token);
                }
                finally
                {
                    canClose = true;
                    if (dialog.IsLoaded)
                    {
                        dialog.Close();
                    }
                }
            }

            dialog.Closing += (_, args) =>
            {
                if (!canClose)
                {
                    args.Cancel = true; // must stay open for now
                    cts.Cancel();
                }
            };

            dialog.Loaded += (_, __) =>
            {
                task = taskRunner();
            };

            dialog.ShowDialog();

            return task.GetAwaiter().GetResult();
        }
    }
}

Problem

I'm trying to create WPF client with IdentityServer authentication. I'm using their `OidcClient` to get logged in. It's whole async while my app is sync and can't be refactored without huge effort. Calling ``` var result = await _oidcClient.LoginAsync(); ``` doesn't wait for the result. Calling `Wait()` or `.Result` causes deadlock. Wrapping it to other `Task.Run` is complaining that the method is not running on UI thread (it opens browser with login dialog). Do you have any idea, how to solve this? Do I need to write custom sync `OidcClient`?

Original source