dotnet/wpf · error · ArgumentException

SR.Collection_BadType

Error message

SR.Collection_BadType

What it means

UIElementCollection.Cast throws ArgumentException when the supplied value is non-null but not a UIElement (the `as UIElement` cast fails). The message names the collection, the value's runtime type, and the expected UIElement type via SR.Collection_BadType.

Solutions

  1. Only add UIElement-derived instances to the collection.
  2. Wrap non-visual data in a suitable container (TextBlock, ContentPresenter, ContentControl).
  3. Type-check with `is UIElement` before calling Add/Insert on the IList path.

Example fix

// before
panel.Children.Add("Hello");
// after
panel.Children.Add(new TextBlock { Text = "Hello" });
Defensive patterns

Strategy: type-guard

Validate before calling

if (value is UIElement uiElement) panel.Children.Add(uiElement);

Type guard

static bool IsUIElement(object o) => o is UIElement;

Try / catch

try { panel.Children.Add(value); }
catch (ArgumentException ex) { /* value was not a UIElement; wrap or convert it */ }

Prevention

When it happens

Trigger: panel.Children.Add(someNonVisualObject), Insert with a business object or string, or IList.Add through a non-generic reference with a wrong-typed item.

Common situations: Mistakenly adding view-model objects or strings to a panel's Children; refactoring that changed what a helper method returns; generic UI-building code that assumes everything added is a control.

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/96fa33f4b328a5ec. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/UIElementCollection.cs:391

        /// <summary>
        /// Method that forwards to VisualCollection.Move
        /// </summary>
        /// <param name="visual"></param>
        /// <param name="destination"></param>
        internal void MoveVisualChild(Visual visual, Visual destination)
        {
            _visualChildren.Move(visual, destination);
        }
		
        private UIElement Cast(object value)
        {
            if (value == null)
                throw new System.ArgumentException(SR.Format(SR.Collection_NoNull, "UIElementCollection"));

            UIElement element = value as UIElement;

            if (element == null)
                throw new System.ArgumentException(SR.Format(SR.Collection_BadType, "UIElementCollection", value.GetType().Name, "UIElement"));

            return element;
        }
		
        #region IList Members

        /// <summary>
        /// Adds an element to the UIElementCollection
        /// </summary>
        int IList.Add(object value)
        {
            return Add(Cast(value));
        }

        /// <summary>
        /// Determines whether an element is in the UIElementCollection.
        /// </summary>
        bool IList.Contains(object value)

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