Show a modal UI in the middle of background operation and continue

This can be achieved by executing modalUI.ShowDialog() asynchronously (upon a future iteration of the UI thread's message loop). The following implementation of ShowDialogAsync does that by using TaskCompletionSource (EAP task pattern) and SynchronizationContext.Post.

Such execution workflow might be a bit tricky to understand, because your asynchronous task is now spread across two separate WPF message loops: the main thread's one and the new nested one (started by ShowDialog). IMO, that's perfectly fine, we're just taking advantage of the async/await state machine provided by C# compiler.

Although, when your task comes to the end while the modal window is still open, you probably want to wait for user to close it. That's what CloseDialogAsync does below. Also, you probably should account for the case when user closes the dialog in the middle of the task (AFAIK, a WPF window can't be reused for multiple ShowDialog calls).

The following code works for me:

using System;using System.Net.Http;using System.Threading;using System.Threading.Tasks;using System.Windows;using System.Windows.Controls;namespace WpfAsyncApp{    public partial class MainWindow : Window    {        public MainWindow()        {            InitializeComponent();            this.Content = new TextBox();            this.Loaded += MainWindow_Loaded;        }        // AsyncWork        async Task AsyncWork(int n, CancellationToken token)        {            // prepare the modal UI window            var modalUI = new Window();            modalUI.Width = 300; modalUI.Height = 200;            modalUI.Content = new TextBox();            try            {                using (var client = new HttpClient())                {                    // main loop                    for (var i = 0; i < n; i++)                    {                        token.ThrowIfCancellationRequested();                        // do the next step of async process                        var data = await client.GetStringAsync("http://www.bing.com/search?q=item" + i);                        // update the main window status                        var info = "#" + i + ", size: " + data.Length + Environment.NewLine;                        ((TextBox)this.Content).AppendText(info);                        // show the modal UI if the data size is more than 42000 bytes (for example)                        if (data.Length < 42000)                        {                            if (!modalUI.IsVisible)                            {                                // show the modal UI window asynchronously                                await ShowDialogAsync(modalUI, token);                                // continue while the modal UI is still visible                            }                        }                        // update modal window status, if visible                        if (modalUI.IsVisible)                            ((TextBox)modalUI.Content).AppendText(info);                    }                }                // wait for the user to close the dialog (if open)                if (modalUI.IsVisible)                    await CloseDialogAsync(modalUI, token);            }            finally            {                // always close the window                modalUI.Close();            }        }        // show a modal dialog asynchronously        static async Task ShowDialogAsync(Window window, CancellationToken token)        {            var tcs = new TaskCompletionSource<bool>();            using (token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: true))            {                RoutedEventHandler loadedHandler = (s, e) =>                    tcs.TrySetResult(true);                window.Loaded += loadedHandler;                try                {                    // show the dialog asynchronously                     // (presumably on the next iteration of the message loop)                    SynchronizationContext.Current.Post((_) =>                         window.ShowDialog(), null);                    await tcs.Task;                    Debug.Print("after await tcs.Task");                }                finally                {                    window.Loaded -= loadedHandler;                }            }        }        // async wait for a dialog to get closed        static async Task CloseDialogAsync(Window window, CancellationToken token)        {            var tcs = new TaskCompletionSource<bool>();            using (token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: true))            {                EventHandler closedHandler = (s, e) =>                    tcs.TrySetResult(true);                window.Closed += closedHandler;                try                {                    await tcs.Task;                }                finally                {                    window.Closed -= closedHandler;                }            }        }        // main window load event handler        async void MainWindow_Loaded(object sender, RoutedEventArgs e)        {            var cts = new CancellationTokenSource(30000);            try            {                // test AsyncWork                await AsyncWork(10, cts.Token);                MessageBox.Show("Success!");            }            catch (Exception ex)            {                MessageBox.Show(ex.ToString());            }        }    }}

[EDITED] Below is a slightly different approach which uses Task.Factory.StartNew to invoke modalUI.ShowDialog() asynchronously. The returned Task can be awaited later to make sure the user has closed the modal dialog.

async Task AsyncWork(int n, CancellationToken token){    // prepare the modal UI window    var modalUI = new Window();    modalUI.Width = 300; modalUI.Height = 200;    modalUI.Content = new TextBox();    Task modalUITask = null;    try    {        using (var client = new HttpClient())        {            // main loop            for (var i = 0; i < n; i++)            {                token.ThrowIfCancellationRequested();                // do the next step of async process                var data = await client.GetStringAsync("http://www.bing.com/search?q=item" + i);                // update the main window status                var info = "#" + i + ", size: " + data.Length + Environment.NewLine;                ((TextBox)this.Content).AppendText(info);                // show the modal UI if the data size is more than 42000 bytes (for example)                if (data.Length < 42000)                {                    if (modalUITask == null)                    {                        // invoke modalUI.ShowDialog() asynchronously                        modalUITask = Task.Factory.StartNew(                            () => modalUI.ShowDialog(),                            token,                            TaskCreationOptions.None,                            TaskScheduler.FromCurrentSynchronizationContext());                        // continue after modalUI.Loaded event                         var modalUIReadyTcs = new TaskCompletionSource<bool>();                        using (token.Register(() =>                             modalUIReadyTcs.TrySetCanceled(), useSynchronizationContext: true))                        {                            modalUI.Loaded += (s, e) =>                                modalUIReadyTcs.TrySetResult(true);                            await modalUIReadyTcs.Task;                        }                    }                }                // update modal window status, if visible                if (modalUI.IsVisible)                    ((TextBox)modalUI.Content).AppendText(info);            }        }        // wait for the user to close the dialog (if open)        if (modalUITask != null)            await modalUITask;    }    finally    {        // always close the window        modalUI.Close();    }}