dotnet/wpf · error · XamlUnexpectedParseException

NonemptyPropertyElement ::= . PROPERTYELEMENT Content?…

Error message

NonemptyPropertyElement ::= . PROPERTYELEMENT Content? ENDTAG.

What it means

P_NonemptyPropertyElement implements NonemptyPropertyElement ::= PROPERTYELEMENT Content? ENDTAG and requires the scanner to be positioned on a PROPERTYELEMENT start token when invoked. If _xamlScanner.NodeType is not ScannerNodeType.PROPERTYELEMENT it throws XamlUnexpectedParseException. This guards the entry state of the nonempty property-element production.

Solutions

Make the property element a proper start tag with matching end tag at the reported location
Do not rewind or partially consume the reader between parses; create a fresh XamlXmlReader per parse
Run an XML well-formedness check on the source before XAML parsing

Example fix

// before (empty tag used where content follows)
<Window.Resources/>
  <SolidColorBrush x:Key="b"/>
// after
<Window.Resources>
  <SolidColorBrush x:Key="b"/>
</Window.Resources>

Defensive patterns

Strategy: validation

Validate before calling

bool matches = System.Text.RegularExpressions.Regex.IsMatch(fragment, "<\\w+[.:][^/>]+>\\s*(<.*?>\\s*)*</\\w+[.:][^>]+>"); if (!matches) throw new InvalidOperationException("Property element must be a proper start/end tag pair");

Try / catch

try { LoadXaml(xamlText); } catch (System.Xaml.XamlParseException ex) { Report(ex.LineNumber, ex.LinePosition, ex.Message); throw; }

Prevention

Ensure containers with children are not self-closed (<X><child/></X>, not <X/><child/>)
Run XSD/designer validation on generated markup
Create a fresh reader per parse; never rewind mid-stream

When it happens

Trigger: P_PropertyElement dispatches to P_NonemptyPropertyElement while the scanner is not on a PROPERTYELEMENT node — malformed markup or a desynchronized reader state at the point where a non-empty property element must begin.

Common situations: XAML where an empty element was written as a container (or vice versa); tool-generated markup with wrong tag forms; manually re-reading a XamlXmlReader after it already advanced past the element.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/805c3788574185a5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Parser/XamlPullParser.cs:371

            }

            yield return Logic_StartMember(_xamlScanner.PropertyElement);
            yield return Logic_EndMember();
            _xamlScanner.Read();
            if (ProvideLineInfo)
            {
                yield return Logic_LineInfo();
            }
        }

        ///////////////////////////
        //  NonemptyPropertyElement ::= PROPERTYELEMENT PropertyContent* ENDTAG
        //
        public IEnumerable<XamlNode> P_NonemptyPropertyElement()
        {
            if (_xamlScanner.NodeType != ScannerNodeType.PROPERTYELEMENT)
            {
                throw new XamlUnexpectedParseException(_xamlScanner, _xamlScanner.NodeType,
                    NonemptyPropertyElementRuleException);
            }

            yield return Logic_StartMember(_xamlScanner.PropertyElement);
            _xamlScanner.Read();
            if (ProvideLineInfo)
            {
                yield return Logic_LineInfo();
            }

            bool doingPropertyContent = true;
            do
            {
                ScannerNodeType nodeType = _xamlScanner.NodeType;
                switch (nodeType)
                {
                case ScannerNodeType.PREFIXDEFINITION:
                case ScannerNodeType.ELEMENT:

View on GitHub (pinned to 81131a70a4)