How can I ensure my async save operation completes on WPF window close?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I want to run an asynchronous operation to save my data when the window is closed, but the UI thread shuts down too quickly so the task never finishes.

Below is my example code:

CSHARP
public partial class MainWindow : Window
{
    public MainWindow()
    {
        Closed += MainWindow_Closed;
        _items = new(Enumerable.Range(0,10));
    }

    private ObservableCollection<int> _items;

    private async void MainWindow_Closed(object? sender, EventArgs e)
    {
        await LongTask();
    }

    private async Task LongTask()
    {
        await Task.Run(() =>
        {
            Trace.WriteLine("LongTask Start");
            Application.Current.Dispatcher.BeginInvoke(() =>
            {
                // do some action in the UI thread
                _items.Clear();
                Trace.WriteLine("Cleared elements");
            });

            Thread.Sleep(5000);
            Trace.WriteLine("LongTask End");
        });
    }
}

The console output never shows 'Cleared elements' or 'LongTask End'. The async task includes UI operations, so using LongTask().GetAwaiter().GetResult() on the main thread would deadlock. I need a solution to ensure the async operation completes properly before the application terminates.

Thanks for the help!

Comments 0

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Answers 3

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3 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is one common solution. Once a window is closed, the UI thread’s dispatcher is already shutting down, so any pending work queued to it (or even work in a background task that needs to interact with the UI) might never complete. One way to ensure your async saving is allowed to finish is to intercept the closing process before the window is gone. For example, you can handle the Closing event (instead of Closed), cancel the closing, await your async operation, and then finally close the window.

Below is an example:

CSHARP
using System.ComponentModel;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        Closing += MainWindow_Closing;
        _items = new(Enumerable.Range(0, 10));
    }

    private ObservableCollection<int> _items;

    private async void MainWindow_Closing(object? sender, CancelEventArgs e)
    {
        // Cancel the close so we can finish our work.
        e.Cancel = true;

        await LongTask();

        // Optionally remove the handler so we don't loop
        Closing -= MainWindow_Closing;

        // Now that the async operation is done, close the window.
        Close();
    }

    private async Task LongTask()
    {
        await Task.Run(() =>
        {
            Trace.WriteLine("LongTask Start");

            // Use Dispatcher.Invoke or BeginInvoke if you need the UI thread.
            Application.Current.Dispatcher.Invoke(() =>
            {
                // Do some action on the UI thread.
                _items.Clear();
                Trace.WriteLine("Cleared elements");
            });

            // Simulate long-running work.
            Thread.Sleep(5000);

            Trace.WriteLine("LongTask End");
        });
    }
}

Explanation

Using the Closing event:
The Closing event occurs before the window is fully closed. By setting e.Cancel = true, you delay the window from closing, giving you time to complete your asynchronous work.
Awaiting your async operation:
Once your operation is finished, you remove the event handler (to prevent re-entry) and call Close() manually so the application can exit gracefully.
Using Dispatcher.Invoke:
Since your async operation needs to update the UI (clearing _items), using Dispatcher.Invoke guarantees that the action is executed on the UI thread before the dispatcher shuts down.

This pattern ensures your async operations complete without running into shutdown-related issues with the UI thread.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by EclipseOrbiter665 • 1 month ago

@IV answer is correct. Just adding some additional context. The key here is a difference in Closing versus Closed.

CSHARP
event CancelEventHandler Closing;
event EventHandler Closed;

In Closed, the process is terminating and any process you start here is doomed to be terminated with the main thread. In Closing, you have the option to intercept the operation and perform additional tasks if necessary.

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Answer by JovianSentinel824 • 1 month ago

As a proof of concept, here's one way you could implement the exceedingly common and routine goal of asynchronously saving your data upon app close. The key here is the _confirmClosure bool, because if the user wants to "save and close" then we're going to:

Disable the UI to prevent any chance of reentry, but cancel the Close().
Asynchronously execute the save.
Then, after awaiting the save, call Close() again, this time with _confirmClosure set to false.

CSHARP
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        Closing += async (sender, e) =>
        {
            if (_confirmClosure)
            {
                switch (
                    MessageBox.Show(
                        "Do you want to save before closing?",
                        "Confirm Exit",
                        MessageBoxButton.YesNoCancel,
                        MessageBoxImage.Question))
                {
                    case MessageBoxResult.Yes:
                        e.Cancel = true;
                        await Dispatcher.BeginInvoke(async () =>
                        {
                            try
                            {
                                Mouse.OverrideCursor = Cursors.Wait;
                                    IsEnabled = false;  // Prevent any more calls.
                                    await DataContext.Save();
                                    _confirmClosure = false;
                                    Close();
                            }
                            finally
                            {
                                Mouse.OverrideCursor = null;
                            }
                        });
                        break;
                    case MessageBoxResult.No:
                        break;

                    case MessageBoxResult.Cancel:
                        e.Cancel = true;
                        break;
                }
            }
        };
    }
    bool _confirmClosure = true;
    new MainPageViewModel DataContext => (MainPageViewModel)base.DataContext;
}

Minimal VM for Test

We'll make a "big" list of 10000 items, then save it to a file upon close.

CSHARP
public class MainPageViewModel : INotifyPropertyChanged
{
    public ObservableCollection<Item> Items { get; } = new ObservableCollection<Item>(
            Enumerable.Range(1, 10000)
            .Select(_=>new Item { Id = _, Name = $"Item {_}" }));

    public event PropertyChangedEventHandler PropertyChanged;

    internal async Task Save()
    {
        await Task.Run(() =>
        {
            var path = Path.Combine(
                Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
                "StackOverflow",
                Assembly.GetEntryAssembly()?.GetName()?.Name ?? "SaveThenClose",
                "list-data.json");
            Directory.CreateDirectory(Path.GetDirectoryName(path));
            var json = JsonConvert.SerializeObject(Items, Formatting.Indented);
            File.WriteAllText(path, json);
        });
        // Add a few seconds for good measure, just for demo purposes.
        await Task.Delay(TimeSpan.FromSeconds(2.5));
    }
}

public class Item
{
    public int Id { get; set; }
    public string? Name { get; set; }
}

XML
<Window x:Class="save_list_then_close.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:save_list_then_close"
        mc:Ignorable="d"
        Title="MainWindow" Height="250" Width="400"
        WindowStartupLocation="CenterScreen">
    <Window.DataContext>
        <local:MainPageViewModel/>
    </Window.DataContext>
    <Grid>
        <DataGrid
            Name="dataGrid"
            ItemsSource="{Binding Items}" 
            AutoGenerateColumns="True" 
            IsReadOnly="True" 
            AlternatingRowBackground="LightGray"/>
    </Grid>
</Window>

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How can I ensure my async save operation completes on WPF window close?

3 Answers

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