dotnet/wpf · error · FileFormatException

SR.XpsValidatingLoaderUriNotInSamePackage

Error message

SR.XpsValidatingLoaderUriNotInSamePackage

What it means

XPSS0ValidatingLoader.Load throws FileFormatException (SR.XpsValidatingLoaderUriNotInSamePackage) when loading a part inside an XPS package whose parent URI belongs to a different package. XPS parts must resolve within the same package as their parent; cross-package references are rejected in strict document mode.

Solutions

Fix the parent/base URI so it points into the same package as the part being loaded.
Remove or correct relationships that cross package boundaries; XPS requires all references within one package.
Regenerate the XPS package with a compliant producer so all URIs share one package URI.
Verify URIs with PackUriHelper.GetPackageUri before loading to confirm they match.

Example fix

// before
pc.BaseUri = new Uri("pack://application:,,,/FixedPage.xaml"); // different package

// after
pc.BaseUri = new Uri("pack://container:,,,/Documents/1/Pages/1.fpage"); // same package as part

Defensive patterns

Strategy: validation

Validate before calling

static bool SamePackage(Uri baseUri, Uri parentUri)
{
    var a = System.IO.Packaging.PackUriHelper.GetPackageUri(baseUri);
    var b = parentUri == null ? a : System.IO.Packaging.PackUriHelper.GetPackageUri(parentUri);
    return a.Equals(b);
}

Type guard

static bool IsSamePackageUri(Uri candidate, Uri reference) =>
    candidate != null && reference != null &&
    System.IO.Packaging.PackUriHelper.GetPackageUri(candidate).Equals(
        System.IO.Packaging.PackUriHelper.GetPackageUri(reference));

Try / catch

try
{
    xamlObject = loader.Load(stream, parentUri, parserContext);
}
catch (FileFormatException ex)
{
    // handle cross-package URI violation: repair BaseUri or re-package
}

Prevention

Keep every XPS part and its references inside a single package
Build BaseUri with PackUriHelper.Create for the correct package
Never mix pack://application and pack://container URIs for XPS parts
Validate package relationships before loading

When it happens

Trigger: Loading XAML in an XPS-document context where PackUriHelper.GetPackageUri(parentUri) differs from the package URI of the part being loaded (XPSS0ValidatingLoader.cs:99), e.g. a BaseUri pointing at pack://application while the part lives in pack://container.

Common situations: XPS documents with relationships targeting parts in an embedded/foreign package; mismatched pack URIs after merging packages; content that mixes application resources and XPS container parts; hand-built pack URIs with the wrong package component.

Understand the failure class

Background: "Invalid URL" / "URL cannot be empty": fix the malformed or missing URL behind request-construction failures — this error's family across 50 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/5c0229f3b3434e11. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/XPSS0ValidatingLoader.cs:99

                
                // Publish the active XPS package URI so that PresentationCore
                // sinks (BitmapDecoder, ColorContext) can enforce same-package
                // containment without depending on PresentationFramework.
                // Nesting-safe: save and restore any previously-active value.
                XpsLoadingContext.ActivePackageUri = packageUri;

                try
                {
                    Package package = PreloadedPackages.GetPackage(packageUri);

                    Uri parentPackageUri = null;

                    if (parentUri != null)
                    {
                        parentPackageUri = PackUriHelper.GetPackageUri(parentUri);
                        if (!parentPackageUri.Equals(packageUri))
                        {
                            throw new FileFormatException(SR.XpsValidatingLoaderUriNotInSamePackage);
                        }
                    }

                    if (package == null)
                    {
                        if (rootElement == null)
                        {
                            XmlReader reader = XmlReader.Create(stream, null, pc);
                            obj = XamlReader.Load(reader, pc, XamlParseMode.Synchronous, true, safeTypes);
                            stream.Close();
                        }

                        return obj;
                    }

                    schema.ValidateRelationships(package, packageUri, partUri, mimeType);

                    if (schema.AllowsMultipleReferencesToSameUri(mimeType))

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