dotnet/wpf · error · InvalidOperationException

SR.TextBoxInvalidChild

Error message

SR.TextBoxInvalidChild

What it means

TextBox's explicit IAddChild.AddChild implementation always throws InvalidOperationException: a TextBox accepts only plain text through IAddChild.AddText, never child elements or objects. XAML that places any element inside a <TextBox> reaches this method and fails with the value's ToString in the message.

Solutions

1. Set plain text via the Text property: <TextBox Text="hello"/> instead of nesting a child.
2. For rich content, use TextBlock, RichTextBox (with FlowDocument), or a ContentControl — not TextBox.
3. If you need placeholder text, use the Tag-style watermark pattern (style with an adorner/VisualBrush) rather than a child element.
4. In code, replace addChild calls with textBox.Text = value.ToString() or textBox.AppendText(...).

Example fix

<!-- before: throws TextBoxInvalidChild -->
<TextBox>
    <TextBlock Text="Enter name"/>
</TextBox>

<!-- after -->
<TextBox Text="Enter name"/>
<!-- or for rich content -->
<RichTextBox>
    <FlowDocument>
        <Paragraph>Enter name</Paragraph>
    </FlowDocument>
</RichTextBox>

// XAML review rule: a <TextBox> element must contain no child elements.
// In code before calling IAddChild.AddChild:
if (value is string s) { textBox.Text = s; }
else if (value is Control or TextBlock) { throw new NotSupportedException("Use TextBox.Text or RichTextBox instead of child elements."); }

static bool IsPlainStringChild(object value) => value is string;

try
{
    ((IAddChild)textBox).AddChild(child);
}
catch (InvalidOperationException ex) when (ex.Message.Contains("AddChild") || ex.TargetSite.Name == "AddChild")
{
    textBox.Text = child?.ToString();
}

When it happens

Trigger: Declaring XAML like <TextBox><TextBlock .../></TextBox> or <TextBox><SomeControl/></TextBox>; calling IAddChild.AddChild(obj) directly in code; any markup parser attempt to add an object child to TextBox.

Common situations: Copying layout from <TextBlock> (which does accept Inlines) into a <TextBox>; nesting a default/placeholder content element inside TextBox instead of using Text or Template parts; programmatic misuse of IAddChild.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Animation_ChildMustBeKeyFrame
• Animation_ChildMustBeKeyFrame
• Animation_NoTextChildren
• Animation_NoTextChildren
• ArgumentException: path

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/7052c118f6129c42. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/TextBox.cs:130

        //
        // -----------------------------------------------------------

        ///<summary>
        /// Called to Add the object as a Child.
        ///</summary>
        ///<param name="value">
        /// Object to add as a child
        ///</param>
        ///<remarks>
        /// This method will always throw InvalidOperationException because
        /// the TextBox only accepts plain text.
        ///</remarks>
        void IAddChild.AddChild(Object value)
        {
            ArgumentNullException.ThrowIfNull(value);

            // TextBox only accepts plain text, via IAddChild.AddText.
            throw new InvalidOperationException(SR.Format(SR.TextBoxInvalidChild, value.ToString()));
        }

        ///<summary>
        /// Called when text appears under the tag in markup.
        ///</summary>
        ///<param name="text">
        /// Text to Add to the Object
        ///</param>
        void IAddChild.AddText(string text)
        {
            ArgumentNullException.ThrowIfNull(text);

            this.TextContainer.End.InsertTextInRun(text);
        }

        /// <summary>
        /// Select the text in the given position and length.
        /// </summary>

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