dotnet/wpf · error · NotSupportedException

SR.ToolBar_InvalidStyle_ToolBarPanel

Error message

SR.ToolBar_InvalidStyle_ToolBarPanel

What it means

ToolBar.FindToolBarPanel looks up the template child named ToolBarPanelTemplateName and requires it to be a ToolBarPanel. If the template contains an element with that name that is not a ToolBarPanel, it throws NotSupportedException (SR.ToolBar_InvalidStyle_ToolBarPanel) naming the actual type. The library throws because ToolBar layout depends on ToolBarPanel's special overflow logic.

Solutions

1. In the custom ToolBar template, make the element named PART_ToolBarPanel an actual ToolBarPanel.
2. Remove the PART_ToolBarPanel-named child entirely if you do not want the default behavior, rather than giving it the wrong type.
3. Compare against the default Aero/Fluent ToolBar template and restore the correct part types.

Example fix

<!-- before -->
<ControlTemplate TargetType="ToolBar">
  <StackPanel x:Name="PART_ToolBarPanel" IsItemsHost="True"/>
</ControlTemplate>

<!-- after -->
<ControlTemplate TargetType="ToolBar">
  <ToolBarPanel x:Name="PART_ToolBarPanel" IsItemsHost="True"/>
</ControlTemplate>

// In template authoring/CI: assert required part types
var panel = template.FindName("PART_ToolBarPanel", toolBar) as ToolBarPanel;
if (panel == null) throw new InvalidOperationException("Template must declare PART_ToolBarPanel as ToolBarPanel");

bool ok = VisualTreeHelper.GetChild(toolBar, i) is ToolBarPanel;

try
{
    toolBar.ApplyTemplate();
}
catch (NotSupportedException ex)
{
    // log template part type error; revert to default ToolBar style
}

When it happens

Trigger: Applying a custom ControlTemplate for ToolBar (via Style/Template or theme change) that includes a child named "PART_ToolBarPanel" (ToolBarPanelTemplateName) whose type is not ToolBarPanel.

Common situations: Hand-written or copied custom ToolBar templates where PART_ToolBarPanel is a StackPanel/Grid/Canvas; renaming or retyping the part during restyling; migrating templates across WPF versions where template parts contract is not respected.

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

• SR.ToolBar_InvalidStyle_ToolBarOverflowPanel
• Animation_ChildMustBeKeyFrame
• Animation_ChildMustBeKeyFrame
• Animation_NoTextChildren
• Animation_NoTextChildren

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ed094b2881b663f4. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ToolBar.cs:596

        /// Gets reference to ToolBar's ToolBarPanel element.
        /// </summary>
        internal ToolBarPanel ToolBarPanel
        {
            get
            {
                if (_toolBarPanel == null)
                    _toolBarPanel = FindToolBarPanel();

                return _toolBarPanel;
            }
        }

        private ToolBarPanel FindToolBarPanel()
        {
            DependencyObject child = GetTemplateChild(ToolBarPanelTemplateName);
            ToolBarPanel toolBarPanel = child as ToolBarPanel;
            if (child != null && toolBarPanel == null)
                throw new NotSupportedException(SR.Format(SR.ToolBar_InvalidStyle_ToolBarPanel, child.GetType()));
            return toolBarPanel;
        }

        /// <summary>
        /// Gets reference to ToolBar's ToolBarOverflowPanel element.
        /// </summary>
        internal ToolBarOverflowPanel ToolBarOverflowPanel
        {
            get
            {
                if (_toolBarOverflowPanel == null)
                    _toolBarOverflowPanel = FindToolBarOverflowPanel();

                return _toolBarOverflowPanel;
            }
        }

        private ToolBarOverflowPanel FindToolBarOverflowPanel()

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