dotnet/wpf · error · FormatException

FormatException(error from XamlTypeName.Parse)

Error message

FormatException(error from XamlTypeName.Parse)

What it means

XamlTypeName.Parse could not parse the given XAML type name string (e.g. 'prefix:Name' or '{ns}Name') into a XamlTypeName, so it returns null internally and the public Parse method surfaces the parser's error text as a FormatException. This library throws it because the input string is not a well-formed XAML type name according to the grammar (namespace prefix, colon, name, optional type arguments).

Solutions

Correct the input string to valid XAML type-name syntax ('prefix:TypeName' or '{namespaceUri}TypeName').
Use XamlTypeName.TryParse (or TryParseList) to validate without exceptions and inspect the out error string.
Verify the IXamlNamespaceResolver actually maps the prefix used in the string to a namespace.
If the string came from serialized output, regenerate it via XamlTypeName.ToString instead of manual concatenation.

Example fix

// before
var name = XamlTypeName.Parse("sys:Int32,"); // FormatException
// after
if (!XamlTypeName.TryParse("sys:Int32", nsResolver, out var xtn, out var error))
{
    Console.WriteLine($"Invalid type name: {error}");
}

Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrWhiteSpace(typeName)) throw new ArgumentException("Type name is empty");
// prefer:
if (!XamlTypeName.TryParse(typeName, nsResolver, out var xtn, out var error))
    throw new FormatException($"Invalid XAML type name '{typeName}': {error}");

Type guard

bool IsValidXamlTypeName(string s, IXamlNamespaceResolver ns) =>
    !string.IsNullOrWhiteSpace(s) && XamlTypeName.TryParse(s, ns, out _, out _);

Try / catch

try { var t = XamlTypeName.Parse(typeName, nsResolver); }
catch (FormatException ex) { /* log ex.Message, surface validation error to user */ }

Prevention

Always prefer TryParse over Parse when input is user- or config-supplied.
Keep prefix-to-namespace mappings in sync with the strings you parse.
Never concatenate type names by hand; round-trip through XamlTypeName.ToString.

When it happens

Trigger: Calling XamlTypeName.Parse(typeName, namespaceResolver) with a malformed string: missing name, unbalanced parentheses in type arguments, unknown/empty prefix that GetNamespace resolves to null, or stray characters.

Common situations: Parsing user-supplied or config-driven type names; hand-constructed strings missing the prefix or using '{'/'}' incorrectly; locale/markup that mangles the syntax; refactoring that changes prefix mappings so the prefix no longer resolves.

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/30f29e1d0026c721. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlTypeName.cs:101

        }

        public static string ToString(IList<XamlTypeName> typeNameList, INamespacePrefixLookup prefixLookup)
        {
            ArgumentNullException.ThrowIfNull(typeNameList);
            ArgumentNullException.ThrowIfNull(prefixLookup);
            return ConvertListToStringInternal(typeNameList, prefixLookup.LookupPrefix);
        }

        public static XamlTypeName Parse(string typeName, IXamlNamespaceResolver namespaceResolver)
        {
            ArgumentNullException.ThrowIfNull(typeName);
            ArgumentNullException.ThrowIfNull(namespaceResolver);

            string error;
            XamlTypeName result = ParseInternal(typeName, namespaceResolver.GetNamespace, out error);
            if (result is null)
            {
                throw new FormatException(error);
            }

            return result;
        }

        public static IList<XamlTypeName> ParseList(string typeNameList, IXamlNamespaceResolver namespaceResolver)
        {
            ArgumentNullException.ThrowIfNull(typeNameList);
            ArgumentNullException.ThrowIfNull(namespaceResolver);

            string error;
            IList<XamlTypeName> result = ParseListInternal(typeNameList, namespaceResolver.GetNamespace, out error);
            if (result is null)
            {
                throw new FormatException(error);
            }

            return result;

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