dotnet/wpf · error · FileFormatException

SR.EncodingNotSupported

Error message

SR.EncodingNotSupported

What it means

PackagingUtilities.PerformInitailReadAndVerifyEncoding reads the XML declaration of a package/piece stream and only accepts UTF-8 or Unicode (UTF-16) encodings explicitly declared via the encoding attribute. Any other declared encoding causes a FileFormatException(SR.EncodingNotSupported), because OPC packaging only supports these two encodings.

Solutions

  1. Re-save or convert the package content to UTF-8 (or UTF-16) encoding.
  2. Change the XML declaration to encoding="UTF-8" in the offending piece.
  3. Fix the upstream generator so it emits UTF-8/UTF-16 encoded package parts.

Example fix

// before
<?xml version="1.0" encoding="ISO-8859-1"?>
// after
<?xml version="1.0" encoding="UTF-8"?>
Defensive patterns

Strategy: validation

Validate before calling

var decl = doc.FirstChild as XmlDeclaration;
string enc = decl?.Encoding;
if (enc != null && !enc.Equals("UTF-8", StringComparison.OrdinalIgnoreCase) && !enc.Equals("UTF-16", StringComparison.OrdinalIgnoreCase))
    throw new InvalidOperationException("package encoding must be UTF-8 or UTF-16");

Try / catch

try { pkg = Package.Open(path); }
catch (FileFormatException ex) when (ex.Message.Contains("encoding")) { /* repair/re-encode file */ }

Prevention

When it happens

Trigger: Opening a package file whose XML declaration carries an encoding attribute other than "UTF-8" or "UTF-16" (e.g. ISO-8859-1, ASCII, Windows-1252).

Common situations: Third-party tools or non-Windows generators producing .zip/.xml package pieces with a different declared encoding; hand-edited XML declarations; files saved by editors that default to other encodings.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/5ce863992e4a0a82. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/IO/Packaging/PackagingUtilities.cs:79

            {
                string encoding;
                encoding = reader.GetAttribute(_encodingAttribute);

                if (encoding != null && encoding.Length > 0)
                {
                    encoding = encoding.ToUpperInvariant();

                    //If a non-empty encoding attribute is present [for example - <?xml version="1.0" encoding="utf-8" ?>]
                    //we check to see if the value is either "utf-8" or utf-16. Only these two values are supported
                    //Note: For Byte order markings that require additional information to be specified in
                    //the encoding attribute in XmlDeclaration have already been ruled out by this check as we allow for
                    //only two valid values.
                    if (string.Equals(encoding, _webNameUTF8, StringComparison.Ordinal)
                        || string.Equals(encoding, _webNameUnicode, StringComparison.Ordinal))
                        return;
                    else
                        //if the encoding attribute has any other value we throw an exception
                        throw new FileFormatException(SR.EncodingNotSupported);
                }
            }

            //if the XmlDeclaration is not present, or encoding attribute is not present, we
            //base our decision on byte order marking. reader.Encoding will take that into account
            //and return the correct value.
            //Note: For Byte order markings that require additional information to be specified in
            //the encoding attribute in XmlDeclaration have already been ruled out by the check above.
            //Note: If not encoding attribute is present or no byte order marking is present the
            //encoding default to UTF8
            if (!(reader.Encoding is UnicodeEncoding || reader.Encoding is UTF8Encoding))
                throw new FileFormatException(SR.EncodingNotSupported);
        }

        /// <summary>
        /// VerifyStreamReadArgs
        /// </summary>
        /// <param name="s">stream</param>

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