dotnet/wpf · error · InvalidOperationException

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

Error message

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

What it means

WispStylusDevice.UpdateOverProperty (WispStylusDevice.cs:436) throws InvalidOperationException(SR.Invalid_IInputElement) when attaching IsEnabledChanged/IsVisibleChanged/IsHitTestVisibleChanged handlers to the new _stylusOver element, which is not a UIElement, ContentElement, or UIElement3D. Mirrors error 700/706 for the WispStylusDevice over-property path.

Solutions

Make hit-testable elements derive from UIElement, ContentElement, or UIElement3D.
Trace _stylusOver.GetType() to find which component created the element.
Validate InputElement.IsValid on elements before they enter the stylus input path.

Example fix

// before
public class Hotspot : FrameworkElement, IInputElement { /* ok */ }
public class Hotspot2 : DependencyObject, IInputElement { /* throws */ }
// after
public class Hotspot2 : 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 { /* change stylus over */ } catch (InvalidOperationException ex) when (ex.Message.Contains("IInputElement")) { /* log offending type and reset stylus over state */ }

Prevention

Test stylus-over behavior with custom controls before shipping
Derive custom input controls from supported base classes
Audit hit-test callbacks for non-standard element types

When it happens

Trigger: ChangeStylusOver sets a new over element that is not one of the three supported input element classes; the else branch of the pattern match throws while wiring the property-change handlers.

Common situations: Custom IInputElement-based controls under the pointer; broken hit-test results returning unsupported elements; version/framework interop where a custom element type slips into stylus state.

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/57d5fe551743093b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/WispStylusDevice.cs:436

                }
                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, _stylusOver.GetType())); 
                }
            }

            // Oddly enough, update the IsStylusOver property first.  This is
            // so any callbacks will see the more-common IsStylusOver property
            // set correctly.
            UIElement.StylusOverProperty.OnOriginValueChanged(oldOver as DependencyObject, _stylusOver as DependencyObject, ref _stylusOverTreeState);

            // Invalidate the IsStylusDirectlyOver property.
            if (oldOver != null)
            {
                o = oldOver as DependencyObject;
                o.SetValue(UIElement.IsStylusDirectlyOverPropertyKey, false); // Same property for ContentElements
            }
            if (_stylusOver != null)
            {
                o = _stylusOver as DependencyObject;
                o.SetValue(UIElement.IsStylusDirectlyOverPropertyKey, true); // Same property for ContentElements

View on GitHub (pinned to 81131a70a4)