dotnet/wpf · error · FileFormatException

SR.XpsValidatingLoaderUnsupportedMimeType

Error message

SR.XpsValidatingLoaderUnsupportedMimeType

What it means

XPSS0ValidatingLoader.Load throws FileFormatException (SR.XpsValidatingLoaderUnsupportedMimeType) when validating an XPS part in strict document mode: the root element name in the markup does not equal the root element expected for the part's declared MIME type. The content contradicts its content type, so the loader refuses it.

Solutions

  1. Fix the part's markup so its root element matches the declared MIME type's schema.
  2. Correct the part's ContentType in [Content_Types].xml or its relationships to reflect the actual markup.
  3. Regenerate the XPS package with a compliant producer instead of hand-editing parts.
  4. Validate the package with XPS tooling before loading.

Example fix

// before: part declared xps-fixeddocument but contains FixedPage markup
<FixedPage xmlns="...">...</FixedPage>

// after: root matches declared content type
<FixedDocument xmlns="...">...</FixedDocument>
Defensive patterns

Strategy: validation

Validate before calling

// peek the first element name and compare to the expected root for the content type
using var reader = XmlReader.Create(stream, XmlReaderSettings { DtdProcessing = DtdProcessing.Ignore });
reader.MoveToContent();
bool rootMatches = reader.Name == "FixedPage"; // expected root for the declared MIME type

Type guard

static bool HasExpectedRoot(Stream s, string expectedRoot)
{
    using var r = XmlReader.Create(s, new XmlReaderSettings { DtdProcessing = DtdProcessing.Ignore });
    r.MoveToContent();
    return r.Name == expectedRoot;
}

Try / catch

try
{
    obj = loader.Load(stream, parentUri, pc, mimeType);
}
catch (FileFormatException ex)
{
    // content type / root mismatch: reject or repair the package part
}

Prevention

When it happens

Trigger: Loading a part inside an XPS package where the XML root element (xpsSchemaValidator.XmlReader.Name) does not match the rootElement string required by the schema for the part's ContentType (XPSS0ValidatingLoader.cs:167), e.g. a part declared as application/vnd.ms-package.xps-fixedpage whose root is not 'FixedPage'.

Common situations: A malformed or hand-edited XPS file where a FixedDocument or ResourceDictionary part contains FixedPage markup or vice versa; wrong MIME type entry in [Content_Types].xml; a renamed root element; tooling that wrote a part with an incorrect content-type mapping.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e9366c8089e0cbf8. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/XPSS0ValidatingLoader.cs:167

                            {
                                if (!validResources.ContainsKey(absTargetUri))
                                {
                                    validResources.Add(absTargetUri, false);
                                }
                            }
                        }
                    }

                    XpsSchemaValidator xpsSchemaValidator = new XpsSchemaValidator(this, schema, mimeType,
                                                                                    stream, packageUri, partUri);
                    _validResources.Push(validResources);
                    if (rootElement != null)
                    {
                        xpsSchemaValidator.XmlReader.MoveToContent();

                        if (!rootElement.Equals(xpsSchemaValidator.XmlReader.Name))
                        {
                            throw new FileFormatException(SR.XpsValidatingLoaderUnsupportedMimeType);
                        }

                        while (xpsSchemaValidator.XmlReader.Read())
                            ;
                    }
                    else
                    {
                        obj = XamlReader.Load(xpsSchemaValidator.XmlReader,
                                    pc,
                                    XamlParseMode.Synchronous, true, safeTypes);
                    }
                    _validResources.Pop();
                }
                finally
                {
                    XpsLoadingContext.ActivePackageUri = previousPackageUri;
                }
            }

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