dotnet/wpf · error · InvalidOperationException

SR.InvalidItemContainer

Error message

SR.InvalidItemContainer

What it means

StatusBar.GetContainerForItemOverride verifies that a generated or returned item container is either a StatusBarItem or a Separator. If PrepareContainerForItemOverride/GetContainerForItemOverride receives anything else, it throws InvalidOperationException naming the StatusBar type and the two valid container types. This guards StatusBar's invariant that every direct child container is one of the two supported types.

Solutions

  1. Return a StatusBarItem (or Separator) from GetContainerForItemOverride, or call base.GetContainerForItemOverride()
  2. If custom content is needed, wrap it inside StatusBarItem.Content rather than replacing the container type
  3. Remove any override or style that changes the ItemContainer for the StatusBar

Example fix

// before
protected override DependencyObject GetContainerForItemOverride()
    => new ListBoxItem(); // wrong container -> throws
// after
protected override DependencyObject GetContainerForItemOverride()
    => new StatusBarItem();
Defensive patterns

Strategy: try-catch

Validate before calling

protected override DependencyObject GetContainerForItemOverride()
{
    var c = base.GetContainerForItemOverride();
    if (c is not StatusBarItem and not Separator)
        throw new InvalidOperationException("StatusBar containers must be StatusBarItem or Separator.");
    return c;
}

Try / catch

try
{
    statusBar.Items.Add(container);
}
catch (InvalidOperationException ex)
{
    // wrap in StatusBarItem before adding
}

Prevention

When it happens

Trigger: Overriding GetContainerForItemOverride/PrepareContainerForItemOverride in a StatusBar subclass and returning a wrong container type (e.g. ContentPresenter, ListBoxItem); a custom ItemsPanel or style forcing a different container; reusing a ListBox-derived container factory for StatusBar.

Common situations: Customizing StatusBar item presentation by subclassing; copying container-generation code from ListBox into StatusBar; templates that implicitly produce a different container.

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/eaae74b047db1ef1. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Primitives/StatusBar.cs:123

        protected override DependencyObject GetContainerForItemOverride()
        {
            object currentItem = _currentItem;
            _currentItem = null;

            if (UsesItemContainerTemplate)
            {
                DataTemplate itemContainerTemplate = ItemContainerTemplateSelector.SelectTemplate(currentItem, this);
                if (itemContainerTemplate != null)
                {
                    object itemContainer = itemContainerTemplate.LoadContent();
                    if (itemContainer is StatusBarItem || itemContainer is Separator)
                    {
                        return itemContainer as DependencyObject;
                    }
                    else
                    {
                        throw new InvalidOperationException(SR.Format(SR.InvalidItemContainer, this.GetType().Name, nameof(StatusBarItem), nameof(Separator), itemContainer));
                    }
                }
            }

            return new StatusBarItem();
        }
        /// <summary>
        /// Prepare the element to display the item.  This may involve
        /// applying styles, setting bindings, etc.
        /// </summary>
        protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
        {
            base.PrepareContainerForItemOverride(element, item);

            Separator separator = element as Separator;
            if (separator != null)
            {
                bool hasModifiers;

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