Decorator marked as PART_ContentHost must have no content.

Error message

Decorator marked as PART_ContentHost must have no content.

What it means

The same template-application logic supports a Decorator (e.g. Border) as PART_ContentHost. If the Decorator's Child is already set, WPF throws NotSupportedException — the decorator must be empty because the internal render scope will be installed as its Child.

Solutions

Leave the PART_ContentHost Decorator empty (no Child in XAML)
Move any visual adjustments to the Decorator's own properties (Padding, Background, CornerRadius)
Wrap with an outer element for extra visuals, keeping the named decorator childless
Start from the default PasswordBox template and modify only outer chrome

Example fix

// before
<Border x:Name="PART_ContentHost">
  <TextBlock Text="inner"/> <!-- throws -->
</Border>
// after
<Border x:Name="PART_ContentHost" Padding="4" Background="White"/>

Defensive patterns

Strategy: validation

Validate before calling

var dec = templateChild as Decorator;
bool ok = dec != null && dec.Child == null; // PART_ContentHost decorator must be empty

Type guard

bool isEmptyDecoratorHost(FrameworkElement fe) => fe is Decorator d && d.Child == null;

Prevention

Never nest children inside the PART_ContentHost Decorator
Use Decorator properties (Padding, Background) for styling instead of nested content
Keep the content-host element as the single child of its container
Diff custom templates against the default one after edits

When it happens

Trigger: Custom PasswordBox template where PART_ContentHost is a Border/Decorator with a child element inside it, evaluated when the template is applied and InitializeRenderScope runs.

Common situations: Decorating the content host with padding/background by nesting content inside the Border; adapting TextBox or ComboBox templates that keep children 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/de4bb80e6e7ef3bf. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/PasswordBox.cs:951

            // Add renderScope as a child of ContentHostTemplateName
            _renderScope = renderScope;
            if (_passwordBoxContentHost is ScrollViewer scrollViewer)
            {
                if (scrollViewer.Content != null)
                {
                    throw new NotSupportedException(SR.TextBoxScrollViewerMarkedAsTextBoxContentMustHaveNoContent);
                }
                else
                {
                    scrollViewer.Content = _renderScope;
                }
            }
            else if (_passwordBoxContentHost is Decorator decorator)
            {
                if (decorator.Child != null)
                {
                    throw new NotSupportedException(SR.TextBoxDecoratorMarkedAsTextBoxContentMustHaveNoContent);
                }
                else
                {
                    decorator.Child = _renderScope; // this may replace old render scope in case of upgrade scenario in TextBox
                }
            }
            else
            {
                // When we implement TextContainer setting via TextView interface
                // all text containing element will become allowed here.
                _renderScope = null;

                // Explicitly not throwing an exception here when content host = null
                // -- designers need us to support no content scenarios
                if (_passwordBoxContentHost != null)
                {
                    _passwordBoxContentHost = null;
                    //  Remove the exception

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