MahApps/MahApps.Metro · error · MissingRequiredTemplatePartException

Template part "{templatePart}" in template for "{target.GetT

Error message

Template part "{templatePart}" in template for "{target.GetType().FullName}" is missing.

What it means

Thrown by MultiSelectionComboBox.OnApplyTemplate when GetTemplateChild('PART_SelectedItemsPresenter') does not return a ListBox. MissingRequiredTemplatePartException (a MahAppsException subtype) indicates the ControlTemplate is missing a required named element. This means the control's default style is not applied or a custom template omits the part.

Source

Thrown at src/MahApps.Metro/Controls/MultiSelectionComboBox/MultiSelectionComboBox.cs:1190

            if (sender is MultiSelectionComboBox)
            {
                e.CanExecute = e.Parameter != null;
            }
        }

        #endregion

        #region Overrides

        public override void OnApplyTemplate()
        {
            this.StopListeningForSelectionChanges();

            base.OnApplyTemplate();

            // Init SelectedItemsPresenter
            var selectedItemsPresenterName = nameof(this.PART_SelectedItemsPresenter);
            this.PART_SelectedItemsPresenter = this.GetTemplateChild(selectedItemsPresenterName) as ListBox ?? throw new MissingRequiredTemplatePartException(this, selectedItemsPresenterName);

            this.PART_SelectedItemsPresenter.MouseLeftButtonUp -= this.PART_SelectedItemsPresenter_MouseLeftButtonUp;
            this.PART_SelectedItemsPresenter.MouseLeftButtonUp += this.PART_SelectedItemsPresenter_MouseLeftButtonUp;
            this.PART_SelectedItemsPresenter.SelectionChanged -= this.PART_SelectedItemsPresenter_SelectionChanged;
            this.PART_SelectedItemsPresenter.SelectionChanged += this.PART_SelectedItemsPresenter_SelectionChanged;

            // Init EditableTextBox
            this.PART_EditableTextBox = this.GetTemplateChild(nameof(this.PART_EditableTextBox)) as TextBox ?? throw new MissingRequiredTemplatePartException(this, nameof(this.PART_EditableTextBox));

            this.PART_EditableTextBox.LostFocus -= this.PART_EditableTextBox_LostFocus;
            this.PART_EditableTextBox.LostFocus += this.PART_EditableTextBox_LostFocus;

            // Init Popup
            this.PART_Popup = this.GetTemplateChild(nameof(this.PART_Popup)) as Popup ?? throw new MissingRequiredTemplatePartException(this, nameof(this.PART_Popup));
            this.PART_PopupListBox = this.GetTemplateChild(nameof(this.PART_PopupListBox)) as ListBox ?? throw new MissingRequiredTemplatePartException(this, nameof(this.PART_PopupListBox));

            if (this.PART_PopupListBox is not null)
            {

View on GitHub (pinned to 72099e310b)

Solutions

Ensure MahApps.Metro resource dictionaries (MahApps.Metro.Styles.Theme) are merged in App.xaml so the default control template is available.
If defining a custom ControlTemplate, include a ListBox named 'PART_SelectedItemsPresenter'.
Check that the correct MahApps version's themes are referenced and no resource override hides the default style.

Example fix

<!-- before: custom template missing parts -->
<ControlTemplate TargetType="controls:MultiSelectionComboBox">
    <TextBox x:Name="PART_EditableTextBox"/>
</ControlTemplate>

<!-- after -->
<ControlTemplate TargetType="controls:MultiSelectionComboBox">
    <Grid>
        <ListBox x:Name="PART_SelectedItemsPresenter"/>
        <TextBox x:Name="PART_EditableTextBox"/>
        <Popup x:Name="PART_Popup">
            <ListBox x:Name="PART_PopupListBox"/>
        </Popup>
    </Grid>
</ControlTemplate>

Defensive patterns

Strategy: validation

Validate before calling

// Verify default style is applied before the control is shown
if (combo.Template == null)
{
    // ensure MahApps resources are merged in App.xaml
    throw new InvalidOperationException("MahApps theme resources not loaded");
}

Try / catch

try
{
    combo.ApplyTemplate();
}
catch (MahAppsException ex) when (ex.Message.Contains("PART_SelectedItemsPresenter"))
{
    // log and fall back to default template
}

Prevention

Merge MahApps.Metro theme dictionaries in App.xaml.
When authoring custom templates, include all required PART_ elements.
Verify the default style key resolves correctly at design time.

When it happens

Trigger: Applying a custom ControlTemplate to MultiSelectionComboBox that lacks the 'PART_SelectedItemsPresenter' ListBox element. Also when the default Generic.xaml style is not merged (missing MahApps resource dictionaries) or when Themes/Generic.xaml is stripped during build.

Common situations: Developer creates a custom Style/ControlTemplate for MultiSelectionComboBox without including all required template parts. MahApps resource dictionaries not properly merged in App.xaml. Control used without its default theme after a NuGet package mismatch.

Related errors

AI-assisted analysis of MahApps/MahApps.Metro@72099e310b (2026-08-13). Data as JSON: /api/errors/622b12e70b616103. Report an issue: GitHub.