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();
}
}
}
Invokeif you're going to use the TPL (and very few situations where you should be using it if you're on older versions of .NET), nor does it do anything to resolve this problem. They're already on the UI thread, and that method needs to be called on the UI thread, so they can neither run it on another thread, nor do they need to do anything to marshal it to the UI thread. – Could