What causes "SR.UnexpectedParameterType" in dotnet/wpf?

Wrap flow content in a FlowDocument: ... .

dotnet/wpf · error · ArgumentException

SR.UnexpectedParameterType

Error message

SR.UnexpectedParameterType

What it means

RichTextBox implements IAddChild, and its only valid child is a single FlowDocument. Calling AddChild (directly or via XAML) with any other object type throws ArgumentException(SR.UnexpectedParameterType) naming the received type and the expected FlowDocument type.

Solutions

Wrap flow content in a FlowDocument: <RichTextBox><FlowDocument><Paragraph>...</Paragraph></FlowDocument></RichTextBox>.
Set the Document property instead of AddChild when constructing in code.
If you want inline text controls, use a TextBox or TextBlock rather than RichTextBox.

Example fix

// before
richTextBox.AddChild(new Paragraph(new Run("hi")));
// after
richTextBox.Document = new FlowDocument(new Paragraph(new Run("hi")));

Defensive patterns

Strategy: type-guard

Validate before calling

if (child is not FlowDocument)
    throw new ArgumentException("RichTextBox child must be a FlowDocument", nameof(child));
richTextBox.AddChild(child);

Type guard

bool IsFlowDocumentChild(object v) => v is FlowDocument;

Try / catch

try { richTextBox.AddChild(value); }
catch (ArgumentException ex) when (ex.Message.Contains("FlowDocument")) { richTextBox.Document = new FlowDocument(); }

Prevention

In XAML always nest flow content inside <FlowDocument> under RichTextBox.
Prefer setting the Document property over IAddChild.AddChild in code.
Remember RichTextBox content model differs from TextBox: only a FlowDocument is accepted.

When it happens

Trigger: IAddChild.AddChild(value) where value is not a FlowDocument — e.g. putting a Paragraph, TextBlock, or arbitrary UIElement as direct XAML content of a RichTextBox.

Common situations: Writing XAML like <RichTextBox><Paragraph/></RichTextBox> (Paragraph must go inside a FlowDocument); copying markup patterns from TextBox/TextBlock; programmatically calling AddChild with a control.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ae699249a5770494. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/RichTextBox.cs:153

        // IAddChild interface
        //
        // -----------------------------------------------------------

        ///<summary>
        /// This method is called to Add the object as a child of the RichTextBox.  This method is used primarily
        /// by the parser; a more direct way of adding a child to a RichTextBox is to use the <see cref="Document" />
        /// property.
        ///</summary>
        ///<param name="value">
        /// The object to add as a child; it must be a UIElement.
        ///</param>
        void IAddChild.AddChild(Object value)
        {
            ArgumentNullException.ThrowIfNull(value);

            if (!(value is FlowDocument))
            {
                throw new ArgumentException(SR.Format(SR.UnexpectedParameterType, value.GetType(), typeof(FlowDocument)), nameof(value));
            }

            if (!_implicitDocument)
            {
                throw new ArgumentException(SR.Format(SR.CanOnlyHaveOneChild, this.GetType(), value.GetType()));
            }

            this.Document = (FlowDocument)value;
        }

        ///<summary>
        /// This method is called by the parser when text appears under the tag in markup.
        /// As RichTextBox do not support text, calling this method has no effect if the text
        /// is all whitespace.  For non-whitespace text, throw an exception.
        ///</summary>
        ///<param name="text">
        /// Text to add as a child.
        ///</param>

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