dotnet/wpf · error · InvalidOperationException

SR.Format(SR.Invalid_IInputElement, oldOver.GetType())

Error message

SR.Format(SR.Invalid_IInputElement, oldOver.GetType())

What it means

WispStylusDevice.UpdateOverProperty (WispStylusDevice.cs:407) throws InvalidOperationException(SR.Invalid_IInputElement) when detaching event handlers (IsEnabledChanged, IsVisibleChanged, IsHitTestVisibleChanged) from oldOver — the previous stylus-over element — and oldOver is not UIElement, ContentElement, or UIElement3D. The over-state bookkeeping expects only those element kinds, so a foreign element in _stylusOver state fails fast.

Solutions

Ensure all elements that can be under the stylus derive from UIElement, ContentElement, or UIElement3D.
Inspect oldOver.GetType() in a debugger to find the source of the foreign element.
Remove custom hit-test code that returns raw IInputElement implementations.

Example fix

// before
class Tile : DependencyObject, IInputElement { }
// after
class Tile : UIElement { }

Defensive patterns

Strategy: type-guard

Validate before calling

static bool TracksStylusOver(object e) => e is UIElement || e is ContentElement || e is UIElement3D;

Type guard

static bool IsValidInputElement(IInputElement e) => e is UIElement || e is ContentElement || e is UIElement3D;

Try / catch

try { /* stylus over transition */ } catch (InvalidOperationException ex) when (ex.Message.Contains("IInputElement")) { /* clear stylus state and fall back to mouse input */ }

Prevention

Keep all hit-testable elements within UIElement/ContentElement/UIElement3D hierarchy
Avoid third-party IInputElement implementations in the visual tree
Debug oldOver.GetType() early when integrating custom input frameworks

When it happens

Trigger: ChangeStylusOver transitions the stylus-over element while the stored previous element is a non-standard IInputElement, usually because it was accepted earlier without validation.

Common situations: Custom IInputElement implementations in the hit-test path; state corruption after visual-tree surgery during stylus moves; third-party input frameworks injecting elements.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b63eec161ea49ca5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/WispStylusDevice.cs:407

                }
                else if (o is ContentElement ce)
                {
                    ce.IsEnabledChanged -= _overIsEnabledChangedEventHandler;

                    // NOTE: there are no IsVisible or IsHitTestVisible properties for ContentElements.
                    //
                    // ce.IsVisibleChanged -= _overIsVisibleChangedEventHandler;
                    // ce.IsHitTestVisibleChanged -= _overIsHitTestVisibleChangedEventHandler;
                }
                else if (o is UIElement3D uie3D)
                {
                    uie3D.IsEnabledChanged -= _overIsEnabledChangedEventHandler;
                    uie3D.IsVisibleChanged -= _overIsVisibleChangedEventHandler;
                    uie3D.IsHitTestVisibleChanged -= _overIsHitTestVisibleChangedEventHandler;
                }
                else
                {
                    throw new InvalidOperationException(SR.Format(SR.Invalid_IInputElement, oldOver.GetType())); 
                }
            }
            if (_stylusOver != null)
            {
                o = _stylusOver as DependencyObject;
                if (o is UIElement uie)
                {
                    uie.IsEnabledChanged += _overIsEnabledChangedEventHandler;
                    uie.IsVisibleChanged += _overIsVisibleChangedEventHandler;
                    uie.IsHitTestVisibleChanged += _overIsHitTestVisibleChangedEventHandler;
                }
                else if (o is ContentElement ce)
                {
                    ce.IsEnabledChanged += _overIsEnabledChangedEventHandler;

                    // NOTE: there are no IsVisible or IsHitTestVisible properties for ContentElements.
                    //
                    // ce.IsVisibleChanged += _overIsVisibleChangedEventHandler;

View on GitHub (pinned to 81131a70a4)