stride3d/stride · error · InvalidOperationException

A part named ' ' must be present in the ControlTemplate…

Error message

A part named '{MessageContainerPartName}' must be present in the ControlTemplate, and must be of type '{typeof(FlowDocumentScrollViewer)}'.

What it means

MarkdownTextBlock.OnApplyTemplate fetches the mandatory template part 'PART_MessageContainer' and requires it to be a FlowDocumentScrollViewer. When the applied ControlTemplate is missing that part (or names it differently / uses another element type), GetTemplateChild returns null and an InvalidOperationException is thrown.

Solutions

Add <FlowDocumentScrollViewer x:Name="PART_MessageContainer"/> to the ControlTemplate
Verify the part name matches MessageContainerPartName exactly (case-sensitive)
Ensure the part element is a FlowDocumentScrollViewer, not a subclass-less substitute like ContentPresenter
Base your custom template on the library's default template for MarkdownTextBlock

Example fix

<!-- before -->
<ControlTemplate TargetType="mtb:MarkdownTextBlock">
  <Border Background="Transparent"/>
</ControlTemplate>
<!-- after -->
<ControlTemplate TargetType="mtb:MarkdownTextBlock">
  <Border Background="Transparent">
    <FlowDocumentScrollViewer x:Name="PART_MessageContainer"/>
  </Border>
</ControlTemplate>

Defensive patterns

Strategy: validation

Validate before calling

// validate template parts before/after ApplyTemplate in your hosting code
var container = markdownTextBlock.Template?.FindName("PART_MessageContainer", markdownTextBlock) as FlowDocumentScrollViewer;
if (container == null) throw new InvalidOperationException("Custom MarkdownTextBlock template is missing PART_MessageContainer (FlowDocumentScrollViewer)");

Type guard

bool HasValidMarkdownTemplate(MarkdownTextBlock c) => c.Template?.FindName("PART_MessageContainer", c) is FlowDocumentScrollViewer;

Try / catch

try { control.ApplyTemplate(); }
catch (InvalidOperationException ex) { log.Error(ex, "Invalid MarkdownTextBlock template; falling back to default style"); control.Style = null; }

Prevention

Never remove PART_-prefixed elements when re-templating
Keep part names and types identical to the default template
Test custom templates by forcing ApplyTemplate in unit tests
Copy templates from the same library version you reference

When it happens

Trigger: Applying a custom ControlTemplate for MarkdownTextBlock that omits the part named by MessageContainerPartName, names it incorrectly, or hosts an element that is not a FlowDocumentScrollViewer.

Common situations: Re-templating the control for theming; upgrading Stride and copying an outdated template; hand-written Styles without the required part; renaming the part x:Name in XAML.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/5d907ce27255ab1e. Report an issue: GitHub.

Appendix: source

Thrown at sources/presentation/Stride.Core.Presentation.Wpf/Controls/MarkdownTextBlock.cs:92

        static MarkdownTextBlock()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(MarkdownTextBlock), new FrameworkPropertyMetadata(typeof(MarkdownTextBlock)));
        }

        public MarkdownTextBlock()
        {
            defaultMarkdown = new Lazy<XamlMarkdown>(() => new XamlMarkdown(this));
        }

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

            messageContainer = GetTemplateChild(MessageContainerPartName) as FlowDocumentScrollViewer;
            if (messageContainer == null)
                throw new InvalidOperationException($"A part named '{MessageContainerPartName}' must be present in the ControlTemplate, and must be of type '{typeof(FlowDocumentScrollViewer)}'.");

            ResetMessage();
        }

        private static void BaseUrlChanged([NotNull] DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var control = d as MarkdownTextBlock;
            if (control == null) throw new ArgumentNullException(nameof(control));

            if (e.NewValue != null)
            {
                control.GetMarkdown().BaseUrl = (string)e.NewValue;
            }
            control.ResetMessage();
        }

        private static void HyperlinkCommandChanged([NotNull] DependencyObject d, DependencyPropertyChangedEventArgs e)
        {

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