dotnet/wpf · error · FileFormatException

FileFormatException(new Uri(fileName…

Error message

FileFormatException(new Uri(fileName, UriKind.RelativeOrAbsolute), message, innerException)

What it means

CompositeFontParser.Fail raises FileFormatException with a caller-supplied message and inner exception plus the font file's BaseURI. Called directly or via FailAttributeValue, it means a value in the composite font document is semantically wrong even though the XML parsed.

Solutions

Read the exception Message and InnerException to identify the offending attribute
Fix or remove the invalid attribute in the .CompositeFont file
Compare against a working .CompositeFont sample for the expected schema
Log the SourceUri property to know which font file failed

Example fix

<!-- before: wrong attribute type -->
<FontFamilyEntry FamilyName="Arial" Weight="Bold" Style="0" />
<!-- after -->
<FontFamilyEntry FamilyName="Arial" Weight="Bold" Style="Normal" />

Defensive patterns

Strategy: try-catch

Validate before calling

// Semantic pre-check example
if (!Enum.TryParse<FontStyle>(styleAttr, out _)) throw new ArgumentException($"Invalid Style: {styleAttr}");

Try / catch

try { ParseCompositeFont(reader); }
catch (FileFormatException e)
{
    log.Error($"Invalid value in font file {e.SourceUri}: {e.Message}", e.InnerException);
}

Prevention

Validate attribute values against the CompositeFont schema before parsing
Log FileFormatException.SourceUri to locate the offending file
Compare against a known-good .CompositeFont template

When it happens

Trigger: An attribute value fails semantic validation during CompositeFont parsing (wrong type, unexpected element, invalid enum-ish value), e.g. a required attribute is missing or has an unparsable value passed to FailAttributeValue.

Common situations: Hand-authored .CompositeFont files with wrong attribute names/types, schema drift between WPF versions, font resources produced by custom tooling emitting invalid values.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/254727e479ab8248. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/FontFace/CompositeFontParser.cs:942

        {
            Fail(SR.Format(SR.CompositeFontMissingAttribute, name));
        }

        /// <summary>
        ///  Fail with a specified error message.
        /// </summary>
        private void Fail(string message)
        {
            Fail(message, null);
        }

        /// <summary>
        /// Fail with a specified error message and inner exception.
        /// </summary>
        private void Fail(string message, Exception innerException)
        {
            string fileName = _reader.BaseURI;
            throw new FileFormatException(new Uri(fileName, UriKind.RelativeOrAbsolute), message, innerException);
        }
        #endregion

        private CompositeFontInfo _compositeFontInfo;

        private XmlReader _reader;

        // XML namespaces for which Mapping processing instructions have been read. For each entry,
        // the key is the XML namespace, and the value is either SystemClrNamespace (if the the 
        // Mapping PI specifies "System" and "MSCORLIB") or String.Empty (any other Mapping).
        private Hashtable _namespaceMap;

        // Type converters for double and XmlLanguage types.
        private TypeConverter _doubleTypeConverter;
        private TypeConverter _xmlLanguageTypeConverter;

        private const string SystemClrNamespace = "System";

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