dotnet/wpf · error · NotSupportedException

SR.TextBoxInvalidTextContainer

Error message

SR.TextBoxInvalidTextContainer

What it means

When the element registered as the TextBox content host turns out not to be a usable text container (it is not a valid render scope such as ScrollViewer/Decorator/Panel as expected), TextBoxBase nulls out the host and throws NotSupportedException. Designers require a no-content scenario to be tolerated, hence the nulling before the throw.

Solutions

  1. Make PART_ContentHost a supported type - ScrollViewer for TextBox, or a Decorator/Panel as the template expects.
  2. Restore the default part structure from the framework's default template.
  3. If a no-content design is intentional, ensure no element is incorrectly named as the content host so designers can render without one.

Example fix

// before
<ContentPresenter x:Name="PART_ContentHost"/> <!-- unsupported type -> throws -->
// after
<ScrollViewer x:Name="PART_ContentHost"/>
Defensive patterns

Strategy: validation

Validate before calling

var host = template.FindName("PART_ContentHost", textBox) as FrameworkElement;
bool isValid = host is ScrollViewer || host is Decorator || host is Panel;

Type guard

static bool IsSupportedContentHost(FrameworkElement fe) => fe is ScrollViewer || fe is Decorator || fe is Panel;

Try / catch

try
{
    textBox.ApplyTemplate();
}
catch (NotSupportedException)
{
    textBox.Template = (ControlTemplate)Application.Current.Resources["DefaultTextBoxTemplate"];
}

Prevention

When it happens

Trigger: Applying a template whose PART_ContentHost element is of an unsupported type (e.g. a raw ContentPresenter or custom FrameworkElement instead of ScrollViewer/Decorator/Panel), so the created text container is invalid.

Common situations: Custom control templates that rename or retype the content host; migrating templates across WPF versions where part requirements changed.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e1c4d26c30456b84. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Primitives/TextBoxBase.cs:1934

                }
                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 (_textBoxContentHost != null)
                {
                    _textBoxContentHost = null;
                    //  Remove the exception
                    throw new NotSupportedException(SR.TextBoxInvalidTextContainer);
                }
            }

            // Attach render scope to TextEditor
            InitializeRenderScope();
        }

        private void ClearContentHost()
        {
            // Detach render scope from TextEditor
            UninitializeRenderScope();

            // Render scope has been created by us,
            // so we need to extract if from visual tree.
            if (_textBoxContentHost is ScrollViewer)
            {
                ((ScrollViewer)_textBoxContentHost).Content = null;
            }

View on GitHub (pinned to 81131a70a4)