dotnet/wpf · error · XamlParseException

brokenRule (SR.UnexpectedToken-based parser rule error)

Error message

brokenRule (SR.UnexpectedToken-based parser rule error)

What it means

MePullParser.Parse processes a markup extension string ('{...}'). If the tokenizer never found a valid markup-extension construct (f.found == false), the accumulated broken-rule description (_brokenRule) is thrown as a XamlParseException, typically carrying SR.UnexpectedToken. This is the generic 'not a parseable markup extension' error.

Solutions

  1. Check the markup-extension string for balanced braces and valid 'Name Args' syntax
  2. Escape literal strings that start with '{' using the '{}' prefix
  3. Validate the extension name exists (e.g. StaticResource, Binding)
  4. Catch XamlParseException and log the tokenizer line/column info

Example fix

// before
button.Content = "{CustomThing}"; // malformed
// after
button.Content = "{StaticResource myThing}"; // or escape: "{}{CustomThing}"
Defensive patterns

Strategy: validation

Validate before calling

bool LooksLikeMarkupExtension(string s)
{
    if (s == null || !s.TrimStart().StartsWith("{")) return true; // literal, fine
    string t = s.Trim();
    if (t == "{" || t == "{}") return true; // escape sequence / empty
    return t.EndsWith("}") && t.Count(c => c=='{') == t.Count(c => c=='}');
}

Try / catch

try { nodes = parser.Parse(meString); }
catch (XamlParseException ex) { /* inspect ex.Message for the broken-rule text */ }

Prevention

When it happens

Trigger: Passing a string starting with '{' that does not match the markup-extension grammar: '{' alone, '{}', '{Foo' with unterminated syntax, or a token sequence violating the ME rules.

Common situations: Dynamically built attribute values with malformed curly syntax, strings intended as literals needing {} escape, truncation during serialization.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a80ab145eb76549e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Parser/MePullParser.cs:68

        //  PositionalArgs ::= (Value (',' PositionalArgs)?) | NamedArg
        //       Value     ::= STRING | QUOTEDMARKUPEXTENSION |MarkupExtension

        public IEnumerable<XamlNode> Parse(string text, int lineNumber, int linePosition)
        {
            _tokenizer = new MeScanner(_context, text, lineNumber, linePosition);
            _originalText = text;
            Found f = new Found();
            NextToken();
            foreach (XamlNode node in P_MarkupExtension(f))
            {
                yield return node;
            }

            if (!f.found)
            {
                string brokenRule = _brokenRule;
                _brokenRule = null;
                throw new XamlParseException(_tokenizer, brokenRule);
            }

            if (_tokenizer.Token != MeTokenType.None)
            {
                throw new XamlParseException(_tokenizer, SR.UnexpectedTokenAfterME);
            }

            if (_tokenizer.HasTrailingWhitespace)
            {
                throw new XamlParseException(_tokenizer, SR.WhitespaceAfterME);
            }
        }

        private void SetBrokenRuleString(string ruleString)
        {
            if (string.IsNullOrEmpty(_brokenRule))
            {
                _brokenRule = SR.Format(SR.UnexpectedToken,

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