dotnet/wpf · error · NotSupportedException

SR.FlowDocumentReaderDecoratorMarkedAsContentHostMustHaveNoC…

Error message

SR.FlowDocumentReaderDecoratorMarkedAsContentHostMustHaveNoContent

What it means

FlowDocumentReader's template marks a Decorator as its content host; OnApplyTemplate requires that this Decorator has no child of its own. If the template author placed content inside the ContentHost Decorator, NotSupportedException (SR.FlowDocumentReaderDecoratorMarkedAsContentHostMustHaveNoContent) is thrown because the reader needs to install its own content there.

Solutions

Remove any children from the ContentHost Decorator in the template
Place additional visuals outside the ContentHost Decorator (e.g. in an outer Grid)
Start from the default FlowDocumentReader template and keep the content host empty

Example fix

// before
<Decorator Name="PART_ContentHost">
  <Border Background="LightGray"/>
</Decorator>
// after
<Grid>
  <Decorator Name="PART_ContentHost"/>
</Grid>

Defensive patterns

Strategy: validation

Validate before calling

var host = template.FindName("PART_ContentHost", reader) as Decorator;
bool ok = host == null || host.Child == null;

Try / catch

try { reader.ApplyTemplate(); }
catch (NotSupportedException) { /* ContentHost decorator has content */ }

Prevention

Keep the content-host Decorator childless in templates
Put extra visuals in outer containers, not inside the host
Re-test custom FlowDocumentReader templates after edits

When it happens

Trigger: Custom ControlTemplate for FlowDocumentReader where the Decorator named as ContentHost (PART_ContentHost) contains child elements; copying the default template and adding decorations inside the content host.

Common situations: Restyling FlowDocumentReader to add borders/overlays directly inside the content-host Decorator.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/7606d3bb62cd8030. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/FlowDocumentReader.cs:98

        /// Build Visual tree
        /// </summary>
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            // Initialize ContentHost.
            // If old ContentHost is enabled, disable it first to ensure appropriate cleanup.
            if (CurrentViewer != null)
            {
                DetachViewer(CurrentViewer);
                _contentHost.Child = null;
            }
            _contentHost = GetTemplateChild(_contentHostTemplateName) as Decorator;
            if (_contentHost != null)
            {
                if (_contentHost.Child != null)
                {
                    throw new NotSupportedException(SR.FlowDocumentReaderDecoratorMarkedAsContentHostMustHaveNoContent);
                }

                SwitchViewingModeCore(ViewingMode);
            }

            // Initialize FindTooBar host.
            // If old FindToolBar is enabled, disable it first to ensure appropriate cleanup.
            if (FindToolBar != null)
            {
                ToggleFindToolBar(false);
            }
            _findToolBarHost = GetTemplateChild(_findToolBarHostTemplateName) as Decorator;
            _findButton = GetTemplateChild(_findButtonTemplateName) as ToggleButton;
        }

        /// <summary>
        /// Whether the master page can be moved to the specified page.
        /// </summary>

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