dotnet/wpf · error · ArgumentException

SR.UnexpectedParameterType

Error message

SR.UnexpectedParameterType

What it means

Decorator's explicit IAddChild.AddChild implementation rejects any value that is not a UIElement. WPF XAML parsing calls this method when assigning the child element, so passing a non-visual object as the Decorator's child triggers ArgumentException with the UnexpectedParameterType message. Decorator can only host visual children, hence the strict type requirement.

Solutions

  1. Ensure the object passed as the Decorator child derives from System.Windows.UIElement
  2. Wrap non-visual content in a UIElement host (e.g. ContentControl or TextBlock) before adding
  3. Fix the XAML so only element syntax (not raw text/data) appears inside the Decorator
  4. If a data item is intended, bind it to Content of a ContentControl instead of adding directly

Example fix

// before
border.AddChild(new MyViewModel());
// after
border.AddChild(new ContentControl { Content = new MyViewModel() });
Defensive patterns

Strategy: type-guard

Validate before calling

if (!(value is UIElement)) throw new ArgumentException(nameof(value));

Type guard

bool IsValidChild(object v) => v is UIElement;

Try / catch

try { ((IAddChild)decorator).AddChild(value); }
catch (ArgumentException ex) { /* value was not a UIElement */ }

Prevention

When it happens

Trigger: Calling IAddChild.AddChild explicitly with a non-UIElement object, or XAML markup that places a non-UIElement (e.g. a string or POCO) directly inside a Border, Viewbox, or other Decorator-derived element.

Common situations: XAML typos placing plain text or data objects as the direct child of a Border; programmatic code casting errors where an object is assumed to be a UIElement but is not; binding a non-visual object into Content where the template uses a Decorator.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a9d36199be0284ad. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Decorator.cs:65

        //  Public Methods
        //
        //-------------------------------------------------------------------

        #region Public Methods

        ///<summary>
        /// This method is called to Add the object as a child of the Decorator.  This method is used primarily
        /// by the parser; a more direct way of adding a child to a Decorator is to use the <see cref="Child" />
        /// property.
        ///</summary>
        ///<param name="value">
        /// The object to add as a child; it must be a UIElement.
        ///</param>
        void IAddChild.AddChild (Object value)
        {
            if (!(value is UIElement))
            {
                throw new ArgumentException (SR.Format(SR.UnexpectedParameterType, value.GetType(), typeof(UIElement)), nameof(value));
            }

            if (this.Child != null)
            {
                throw new ArgumentException(SR.Format(SR.CanOnlyHaveOneChild, this.GetType(), value.GetType()));
            }

            this.Child = (UIElement)value;
        }

        ///<summary>
        /// This method is called by the parser when text appears under the tag in markup.
        /// As Decorators do not support text, calling this method has no effect if the text
        /// is all whitespace.  For non-whitespace text, throw an exception.
        ///</summary>
        ///<param name="text">
        /// Text to add as a child.
        ///</param>

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