dotnet/wpf · error · XamlUnexpectedParseException

StartElement ::= . ELEMENT DIRECTIVE*.

Error message

StartElement ::= . ELEMENT DIRECTIVE*.

What it means

XamlPullParser.P_StartElement implements `StartElement ::= ELEMENT DIRECTIVE*`. It throws XamlUnexpectedParseException when the scanner's NodeType is not ELEMENT, i.e. the parser was asked to produce a start-object node sequence while the current scanner node is not a begin-element token. This guards the grammar contract for open-element tokens (<Tag>) handled by P_Element's second branch.

Solutions

  1. Fix the scanner so begin-element tokens set NodeType = ELEMENT (self-closing ones must be EMPTYELEMENT).
  2. Route parsing through P_Element rather than calling P_StartElement directly.
  3. Verify the input is well-formed XML and restart the reader if a previous exception desynchronized state.
  4. If a framework bug is suspected, reproduce minimally and upgrade the WPF/System.Xaml package.

Example fix

// before
var nodes = parser.P_StartElement(); // NodeType was not ELEMENT

// after
var nodes = parser.P_Element(); // validates EMPTYELEMENT vs ELEMENT itself
Defensive patterns

Strategy: try-catch

Try / catch

try { return XamlReader.Parse(xaml); } catch (XamlUnexpectedParseException ex) { Log($"Expected ELEMENT node, got {ex.NodeType}"); return null; }

Prevention

When it happens

Trigger: P_Element's start-element branch entered while the scanner node is not ELEMENT — from a custom scanner emitting wrong node types, or from invoking P_StartElement outside the P_Element flow; also from corrupted scanner state on malformed input.

Common situations: Custom reader/scanner implementations failing to classify '<Tag>' as ScannerNodeType.ELEMENT; tests calling P_StartElement directly without positioning the scanner; desynchronized scanner after prior parse errors.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/8618bcb55fdd92fb. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Parser/XamlPullParser.cs:176

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

            yield return Logic_EndOfAttributes();
            yield return Logic_EndObject();
        }

        ///////////////////////////
        //  StartElement ::= ELEMENT DIRECTIVE*
        //
        public IEnumerable<XamlNode> P_StartElement()
        {
            if (_xamlScanner.NodeType != ScannerNodeType.ELEMENT)
            {
                throw new XamlUnexpectedParseException(_xamlScanner, _xamlScanner.NodeType,
                    StartElementRuleException);
            }

            yield return Logic_StartObject(_xamlScanner.Type, _xamlScanner.Namespace);
            _xamlScanner.Read();
            if (ProvideLineInfo)
            {
                yield return Logic_LineInfo();
            }

            while (_xamlScanner.NodeType == ScannerNodeType.DIRECTIVE)
            {
                // Directives are processed exactly like Attributes.
                foreach (XamlNode node in LogicStream_Attribute())
                {
                    yield return node;
                }

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