dotnet/wpf · error · ArgumentException

The element must be a DependencyObject

Error message

The element must be a DependencyObject

What it means

WindowChrome.IsHitTestVisibleInChrome is an attached property stored on DependencyObjects, but the getter accepts the broader IInputElement type for convenience. When the passed element implements IInputElement but is not a DependencyObject, the cast fails and ArgumentException is thrown, since there is nowhere to read the attached value.

Solutions

1. Pass a WPF element derived from DependencyObject (UIElement, FrameworkElement, ContentElement).
2. Add a check: if (element is DependencyObject) before calling.
3. If the element is truly not a DependencyObject, the attached property cannot be used — track the flag externally instead.
4. Call the getter with the actual Control/element instance rather than an interface wrapper.

Example fix

// before
IInputElement target = GetInputTarget();
bool v = WindowChrome.GetIsHitTestVisibleInChrome(target); // may throw
// after
if (target is DependencyObject dobj)
    bool v = WindowChrome.GetIsHitTestVisibleInChrome(dobj);

if (element == null) throw new ArgumentNullException(nameof(element));
if (element is not DependencyObject)
    throw new ArgumentException("Element must be a DependencyObject", nameof(element));

bool CanUseAttachedChromeProps(IInputElement e) => e is DependencyObject;

try { var v = WindowChrome.GetIsHitTestVisibleInChrome(el); }
catch (ArgumentException) { /* element is not a DependencyObject */ }

When it happens

Trigger: GetIsHitTestVisibleInChrome(myElement) where myElement implements IInputElement (e.g. a custom raw input target or interop adapter) but does not derive from DependencyObject (UIElement/ContentElement).

Common situations: Passing wrapper/adapter objects from interop scenarios; passing non-visual input sources; generic code typed to IInputElement that assumes WPF elements.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

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• array
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AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/10b56f8eed01214e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Shell/WindowChrome.cs:118

            Verify.IsNotNull(window, "window");
            window.SetValue(WindowChromeProperty, chrome);
        }

        public static readonly DependencyProperty IsHitTestVisibleInChromeProperty = DependencyProperty.RegisterAttached(
            "IsHitTestVisibleInChrome",
            typeof(bool),
            typeof(WindowChrome),
            new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.Inherits));

        [SuppressMessage("Microsoft.Design", "CA1062:Validate arguments of public methods", MessageId = "0")]
        [SuppressMessage("Microsoft.Design", "CA1011:ConsiderPassingBaseTypesAsParameters")]
        public static bool GetIsHitTestVisibleInChrome(IInputElement inputElement)
        {
            Verify.IsNotNull(inputElement, "inputElement");
            var dobj = inputElement as DependencyObject;
            if (dobj == null)
            {
                throw new ArgumentException("The element must be a DependencyObject", nameof(inputElement));
            }
            return (bool)dobj.GetValue(IsHitTestVisibleInChromeProperty);
        }

        [SuppressMessage("Microsoft.Design", "CA1062:Validate arguments of public methods", MessageId = "0")]
        [SuppressMessage("Microsoft.Design", "CA1011:ConsiderPassingBaseTypesAsParameters")]
        public static void SetIsHitTestVisibleInChrome(IInputElement inputElement, bool hitTestVisible)
        {
            Verify.IsNotNull(inputElement, "inputElement");
            var dobj = inputElement as DependencyObject;
            if (dobj == null)
            {
                throw new ArgumentException("The element must be a DependencyObject", nameof(inputElement));
            }
            dobj.SetValue(IsHitTestVisibleInChromeProperty, hitTestVisible);
        }

        public static readonly DependencyProperty ResizeGripDirectionProperty = DependencyProperty.RegisterAttached(

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