dotnet/wpf · error · FileFormatException

FileFormatException(new Uri(_reader.BaseURI…

Error message

FileFormatException(new Uri(_reader.BaseURI, UriKind.RelativeOrAbsolute), x)

What it means

FailNotWellFormed wraps any XmlException raised while reading a composite font (.CompositeFont) into a FileFormatException carrying the source URI and the original XML exception as InnerException. It signals that the font description file is not well-formed XML.

Solutions

Open the .CompositeFont file in an XML editor / xmllint to find the malformed region
Inspect InnerException for the exact line/position of the XML error
Restore the file from a known-good source or regenerate it
Verify the file encoding matches the declared XML encoding

Example fix

<!-- before: malformed -->
<FontFamily>
  <FontFamilyEntry FamilyName="Arial"
<!-- after: well-formed -->
<FontFamily>
  <FontFamilyEntry FamilyName="Arial" />
</FontFamily>

Defensive patterns

Strategy: try-catch

Validate before calling

// Pre-validate with XDocument
document = System.Xml.Linq.XDocument.Load(path, LoadOptions.None); // throws XmlException early if malformed

Type guard

bool IsLoadableXml(string path) { try { XDocument.Load(path); return true; } catch { return false; } }

Try / catch

try { LoadFontFamily(uri); }
catch (FileFormatException e) when (e.InnerException is System.Xml.XmlException xe)
{
    log.Error($"Malformed CompositeFont XML in {e.SourceUri} at line {xe.LineNumber}: {xe.Message}");
}

Prevention

Run xmllint over .CompositeFont files in CI
Keep font resources under source control, never hand-edit deployed copies
Preserve file encoding and BOM during build steps

When it happens

Trigger: XmlReader encounters malformed XML (unclosed tags, invalid characters, encoding errors) while CompositeFontParser walks the document — e.g. when WPF loads a font family from a CompositeFont resource.

Common situations: Truncated .CompositeFont file, wrong encoding/BOM, XML edited by hand or by a build step that broke escaping, font resource served over a stream that got cut off.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/26500612c2a1d33f. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/FontFace/CompositeFontParser.cs:872

        /// <summary>
        /// Make sure the minimum required information is specified.
        /// </summary>
        private void VerifyCompositeFontInfo()
        {
            if (_compositeFontInfo.FamilyMaps.Count == 0)
                Fail(SR.Format(SR.CompositeFontMissingElement, FamilyMapElement));

            if (_compositeFontInfo.FamilyNames.Count == 0)
                Fail(SR.Format(SR.CompositeFontMissingElement, StringElement));
        }

        /// <summary>
        /// Fail because of an XML exception.
        /// </summary>
        private void FailNotWellFormed(Exception x)
        {
            throw new FileFormatException(new Uri(_reader.BaseURI, UriKind.RelativeOrAbsolute), x);
        }

        /// <summary>
        /// Fail because of an incorrect attribute value.
        /// </summary>
        private void FailAttributeValue()
        {
            Fail(SR.Format(
                SR.CompositeFontAttributeValue1,
                _reader.LocalName));
        }

        /// <summary>
        /// Fail because of an incorrect attribute value with an inner exception.
        /// </summary>
        private void FailAttributeValue(Exception x)
        {
            Fail(SR.Format(

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