ElementNotAvailableException thrown when clicking navigation button in WPF Calendar control on .NET 10

[Znt6]

Description

ElementNotAvailableException is thrown when pressing the Calendar control's navigation button.

This issue occurs in both Debug and Release builds. However, automatic page-turning triggered by clicking a date outside the current month does not appear to throw this exception. After automatic page-turning, manually pressing the navigation button does not throw the exception 100% of the time, but it still occurs intermittently.

I have tested this on two devices, both running .NET 10.0.301, with Windows versions 10.0.26200.7840 and 10.0.26200.8655 respectively.

Reproduction Steps

The application crashes and throws an exception when pressing the Calendar control.

Stack trace:

Unhandled exception. System.Windows.Automation.ElementNotAvailableException: 元素不存在，或者已经虚拟化；如果支持 VirtualizedItem 模式，请使用该模式。
at System.Windows.Automation.Peers.DateTimeAutomationPeer.ThrowElementNotAvailableException()
at System.Windows.Automation.Peers.DateTimeAutomationPeer.GetChildrenCore()
at System.Windows.Automation.Peers.AutomationPeer.EnsureChildren()
at System.Windows.Automation.Peers.AutomationPeer.GetChildren()
at System.Windows.Automation.Peers.AutomationPeer.isDescendantOf(AutomationPeer parent)
at System.Windows.Automation.Peers.AutomationPeer.isDescendantOf(AutomationPeer parent)
at System.Windows.Automation.Peers.AutomationPeer.isDescendantOf(AutomationPeer parent)
at System.Windows.Automation.Peers.AutomationPeer.ValidateConnected(AutomationPeer connectedPeer)
at System.Windows.Automation.Peers.AutomationPeer.AutomationPeerFromInputElement(IInputElement focusedElement)
at System.Windows.Automation.Peers.AutomationPeer.RaiseFocusChangedEventHelper(IInputElement newFocus)
at System.Windows.Input.KeyboardDevice.ChangeFocus(DependencyObject focus, Int32 timestamp)
at System.Windows.Input.KeyboardDevice.Focus(DependencyObject focus, Boolean askOld, Boolean askNew, Boolean forceToNullIfFailed)
at System.Windows.Input.KeyboardDevice.Focus(IInputElement element)
at System.Windows.UIElement.Focus()
at System.Windows.Input.KeyboardNavigation.Navigate(DependencyObject currentElement, TraversalRequest request, ModifierKeys modifierKeys, DependencyObject firstElement, Boolean fromProcessInputTabKey)
at System.Windows.FrameworkElement.MoveFocus(TraversalRequest request)
at System.Windows.Controls.Primitives.CalendarItem.FocusDate(DateTime date)
at System.Windows.Controls.Calendar.MoveDisplayTo(Nullable`1 date)
at System.Windows.Controls.Calendar.OnNextClick()
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.Controls.Primitives.ButtonBase.OnClick()
at System.Windows.Controls.Button.OnClick()
at System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
at System.Windows.Input.InputManager.ProcessStagingArea()
at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at Chapter06.App.Main()

Expected behavior

The Calendar control navigates pages normally.

Actual behavior

The application crashes and throws an ElementNotAvailableException.

Regression?

.NET 10

Known Workarounds

Workaround:

public class FCalendar : System.Windows.Controls.Calendar
{
    protected override AutomationPeer OnCreateAutomationPeer() => null;
}

Impact

No response

Configuration

No response

Other information

No response

[gpaskaleva-msft]

Thanks for the detailed stack trace! I reproduced your exact crash with a small harness that connects a UI Automation client (standing in for Narrator/Inspect) and clicks the next-month button. The crash needs a UIA client connected and the newly-focused day button's peer to
not already be connected, so ValidateConnected walks the tree and hits a stale DateTimeAutomationPeer.

One surprise: in my harness this repros identically on .NET 8, 9, 10, and 11 — I couldn't get it to behave differently between versions. So I'd like to confirm the ".NET 10 regression" classification. Could you help with:

1. A minimal repro project (.csproj + XAML + code-behind) — ideally just a Window with a Calendar. The stack shows Chapter06.App.Main(); a trimmed standalone sample would be the most helpful thing.
2. Which accessibility tool is running when it crashes (Narrator, Inspect, Accessibility Insights, a UIA test framework…)? Does it still crash with none open?
3. Exact steps from a fresh launch — first click, or only after selecting/clicking a date? How often does it happen?
4. Did you run the same app on .NET 8/9 and confirm it doesn't repro there (same machine, same tool running)? This is the key point for confirming a .NET 10 regression.
5. Is the Calendar customized (Style/Template, DisplayMode, SelectedDate binding) or default?

Thanks again!

[Znt6]

XAML:

<Window x:Class="WpfApp1.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:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Calendar></Calendar>
    </Grid>
</Window>

CS:

using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp1;

/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
}

After my testing, I found that the exception is caused by WindowSill — a UIA client application running on my machine. With this application enabled, the crash occurs on both .NET 8 and .NET 10.