lepoco/wpfui · error · NullReferenceException

Element {nameof(TemplateElementToggleButton)} of type {typeo

Error message

Element {nameof(TemplateElementToggleButton)} of type {typeof(ToggleButton)} not found in {typeof(SplitButton)}

What it means

During OnApplyTemplate, SplitButton looks up the named template part 'PART_SplitButtonToggleButton' and requires it to be a ToggleButton. If GetTemplateChild returns null or a non-ToggleButton, a NullReferenceException is thrown. SplitButton cannot function without this part because it drives the drop-down toggle behaviour, so a custom ControlTemplate missing it is treated as a fatal template contract violation.

Source

Thrown at src/Wpf.Ui/Controls/SplitButton/SplitButton.cs:159

    }

    /// <summary>This method is invoked when the <see cref="IsDropDownOpenProperty"/> changes.</summary>
    /// <param name="currentValue">The new value of <see cref="IsDropDownOpenProperty"/>.</param>
    protected virtual void OnIsDropDownOpenChanged(bool currentValue) { }

    /// <inheritdoc />
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        if (GetTemplateChild(TemplateElementToggleButton) is ToggleButton toggleButton)
        {
            SplitButtonToggleButton = toggleButton;
            AttachToggleButtonClick();
        }
        else
        {
            throw new NullReferenceException(
                $"Element {nameof(TemplateElementToggleButton)} of type {typeof(ToggleButton)} not found in {typeof(SplitButton)}"
            );
        }

        if (GetTemplateChild(TemplateElementToggle) is Border toggleBorder)
        {
            _splitButtonToggleBorder = toggleBorder;
        }
    }

    /// <summary>
    /// Triggered when the control is unloaded. Releases resource bindings.
    /// </summary>
    protected virtual void ReleaseTemplateResources()
    {
        if (SplitButtonToggleButton != null)
        {
            SplitButtonToggleButton.PreviewMouseLeftButtonUp -=

View on GitHub (pinned to ffebacd610)

Solutions

1. Include <ToggleButton x:Name="PART_SplitButtonToggleButton" /> in your custom ControlTemplate for SplitButton.
2. If you do not need a custom template, remove your Style/Template so the default one is used.
3. Update your template to match the template-part contract declared on SplitButton for the version you target.

Example fix

<!-- before: custom template missing the toggle -->
<ControlTemplate TargetType="{x:Type ui:SplitButton}">
    <Border x:Name="PrimaryBorder" />
</ControlTemplate>

<!-- after -->
<ControlTemplate TargetType="{x:Type ui:SplitButton}">
    <StackPanel Orientation="Horizontal">
        <ContentPresenter />
        <ToggleButton x:Name="PART_SplitButtonToggleButton" />
    </StackPanel>
</ControlTemplate>

// Validate template before applying in design/test
var hasPart = myTemplate.LoadContent() is FrameworkElement fe
    && fe.FindName("PART_SplitButtonToggleButton") is ToggleButton;
if (!hasPart) throw new InvalidOperationException("Template missing PART_SplitButtonToggleButton");

bool TemplateHasRequiredPart(ControlTemplate tpl, Type owner) =>
    tpl.LoadContent() is FrameworkElement root && root.FindName("PART_SplitButtonToggleButton") is ToggleButton;

try { ApplyTemplate(); }
catch (NullReferenceException ex) when (ex.Message.Contains("PART_SplitButtonToggleButton"))
{
    _logger.LogError(ex, "Custom SplitButton template is missing the required toggle part.");
}

When it happens

Trigger: Replacing the default SplitButton ControlTemplate with a custom one that omits the ToggleButton named PART_SplitButtonToggleButton, or names a control of a different type with that name.

Common situations: Restyling SplitButton and forgetting to carry over the required template part; renaming the part during a version upgrade; copying a template from a different control.

Related errors

• Template part '{name}' is not found or is not of type {typeo
• Unable to find the base directory of the application.
• Unable to determine the window source.
• Cannot change DialogHostEx while the dialog is being shown.
• DialogHost was not set

AI-assisted analysis of lepoco/wpfui@ffebacd610 (2026-08-13). Data as JSON: /api/errors/2670f609516ae3dd. Report an issue: GitHub.