dotnet/wpf · error · MappingParseError

MappingParseError(_scanner.Start, MappingScanner.Ident…

Error message

MappingParseError(_scanner.Start, MappingScanner.Ident, _token, ident)

What it means

A MappingParseError thrown by XamlReaderHelper's private Error(ident) helper when the mapping scanner fails to parse an identifier in a mapping/prefix declaration (e.g. xmlns:prefix='clr-namespace:...' processing instructions). The XAML loader could not read the expected identifier token while processing mapping directives, so it aborts parsing.

Solutions

Fix the mapping/namespace declaration in the XAML so it contains a valid identifier (e.g. clr-namespace:MyNs;assembly=MyAssembly).
Check the exact position reported by the error (scanner Start/token) to find the malformed character or missing token.
If XAML is generated at runtime, validate/escape namespace strings before embedding them in the XAML.
If using the Mapping PI, ensure required parts (ClrNamespace, Assembly) are present and well-formed.

Example fix

// before
<?Mapping XmlNamespace='ui' ClrNamespace='My.App.UI;' ?>
// after
<?Mapping XmlNamespace='ui' ClrNamespace='My.App.UI' Assembly='My.App' ?>

Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrWhiteSpace(xaml) || !System.Text.RegularExpressions.Regex.IsMatch(xaml, "clr-namespace:[A-Za-z_][A-Za-z0-9_.]*")) throw new FormatException("Mapping declaration is malformed; expected clr-namespace:Name;assembly=Asm");

Try / catch

try { return (T)XamlReader.Parse(xaml); } catch (XamlParseException ex) { log.Error($"Mapping error at {ex.LinePosition}: {ex.Message}"); throw; }

Prevention

Validate mapping PIs and xmlns declarations before loading.
Use fully qualified clr-namespace with assembly= clause.
Test generated XAML against the real parser in CI.

When it happens

Trigger: Calling XamlReader.Load/XamlReader parse flows on XAML whose mapping PI (<?Mapping ...?>) or clr-namespace/xmlns declarations contain a malformed or missing identifier where the MappingScanner expected an Ident token.

Common situations: Hand-written or generated XAML with typos in clr-namespace names, missing assembly= parts, stray characters in xmlns mappings, or programmatically constructed XAML strings produced by tools emitting invalid mapping syntax.

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/34084993afec6add. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlReaderHelper.cs:2713

                    Expected = expected;
                    Received = received;
                }

                public MappingParseError(int offset, char expected, char received, string ident)
                    : this(offset, expected, received)
                {
                    Ident = ident;
                }
            }

            private void Error(char token)
            {
                throw new MappingParseError(_scanner.Start, token, _token);
            }

            private void Error(string ident)
            {
                throw new MappingParseError(_scanner.Start, MappingScanner.Ident, _token, ident);
            }
        }
        #endregion

        private string ResolveAttributeNamespaceURI(string prefix, string name, string parentURI)
        {
            string attribNamespaceURI;
            if(!String.IsNullOrEmpty(prefix))
            {
                attribNamespaceURI = XmlReader.LookupNamespace(prefix);
            }
            else
            {
                // if the prefix was "" then
                // 1) normal properties resolve to the parent Tag namespace.
                // 2) Attached properties resolve to the "" default namespace.
#if !NETFX
                if (!name.Contains('.'))

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