dotnet/wpf · error · XmlException

XmlException

Error message

XmlException

What it means

XmlCompatibilityReader.Error is the internal helper that raises XmlException for any validation failure while reading compatibility-mode XAML (checking for unsupported nodes, bad syntax, or disallowed constructs under the current CompatibilityScope). It formats the message with the reader's current line/line position (defaulting to 1:1 when the underlying reader exposes no IXmlLineInfo), so every compatibility-check failure surfaces as System.Xml.XmlException.

Solutions

  1. Read the XmlException message and reported line/position and fix the markup at that exact location.
  2. Validate the XAML/XML file with an XML editor or linter before loading it.
  3. Check that the document follows the compatibility mode's allowed constructs (no unsupported nodes/attributes for that scope).
  4. If the error reports line 1 position 1, the underlying reader lacked line info — inspect the whole document for encoding issues (BOM, invalid characters).
  5. Re-encode the file as UTF-8 and re-save it, then retry the load.

Example fix

// before (mismatched tag)
<StackPanel>
  <Button>Click</TextBlock>
</StackPanel>

// after
<StackPanel>
  <Button>Click</Button>
</StackPanel>
Defensive patterns

Strategy: try-catch

Validate before calling

// Validate XML well-formedness before handing it to the XAML reader
try
{
    using var reader = XmlReader.Create(new StringReader(xamlText));
    while (reader.Read()) { }
}
catch (XmlException ex)
{
    throw new FormatException($"Invalid markup at line {ex.LineNumber}, position {ex.LinePosition}: {ex.Message}", ex);
}

Type guard

static bool IsWellFormedXml(string xml)
{
    try { using var r = System.Xml.XmlReader.Create(new System.IO.StringReader(xml)); while (r.Read()) { } return true; }
    catch (System.Xml.XmlException) { return false; }
}

Try / catch

try
{
    XamlReader.Load(xamlStream);
}
catch (System.Xml.XmlException ex)
{
    Console.Error.WriteLine($"Markup error at {ex.LineNumber}:{ex.LinePosition}: {ex.Message}");
}

Prevention

When it happens

Trigger: Any malformed or unsupported XML construct read by XmlCompatibilityReader: invalid element/attribute names, unsupported DTD or processing instructions, mismatched tags, or content not allowed in compatibility mode — each Error(...) call site funnels here to throw XmlException with line info, XmlCompatibilityReader.cs:1476-1482.

Common situations: Hand-edited XAML with typos or mismatched tags; XML saved with unsupported encodings or constructs (undefined entities, invalid characters); documents written for a different XAML compatibility level; copy-pasted markup containing invalid syntax.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9d2f2ed5db5a175a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/Shared/System/Windows/Markup/XmlCompatibilityReader.cs:1481

        /// functionality is supported, but not implemented
        /// </summary>
        private void HandlePreserveAttributes(int elementDepth)
        {
            PushScope(elementDepth);

            foreach (NamespaceElementPair pair in ParseContentToNamespaceElementPair(Reader.Value, _preserveAttributes))
            {
                Scope.PreserveAttribute(pair.namespaceName, pair.itemName);
            }
        }

        /// <summary>
        /// helper method to generate an exception
        /// </summary>
        private void Error(string message, params object[] args)
        {
            IXmlLineInfo info = Reader as IXmlLineInfo;
            throw new XmlException(string.Format(CultureInfo.InvariantCulture, message, args), null, info is null ? 1 : info.LineNumber,
                info is null ? 1 : info.LinePosition);
        }
        #endregion Private Methods

        #region Private Properties
        private CompatibilityScope Scope
        {
            get
            {
                return _compatibilityScope;
            }
        }

        private string AlternateContent
        {
            get
            {
                if (_alternateContent is null)

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