dotnet/wpf · error · FileFormatException

SR.XpsValidatingLoaderMoreThanOneThumbnailInPackage

Error message

SR.XpsValidatingLoaderMoreThanOneThumbnailInPackage

What it means

The XPS validating loader enforces that an XPS package contains at most one thumbnail part. While enumerating relationships of type thumbnail on a part, a second relationship was found, so the loader throws FileFormatException to reject the non-conformant package.

Solutions

  1. Open the package, enumerate the .rels files, and delete all but one thumbnail relationship.
  2. Regenerate the XPS with a conformant producer that emits exactly one thumbnail.
  3. Strip the extra thumbnail rels with a script before loading (Package.DeleteRelationship).
  4. Catch FileFormatException and surface a 'document is malformed' message to the user.

Example fix

// before: two thumbnail rels in _rels/.rels
<Relationship Type="http://schemas.microsoft.com/xps/2005/06/thumbnail" Target="thumbnails/one.png" Id="R1"/>
<Relationship Type="http://schemas.microsoft.com/xps/2005/06/thumbnail" Target="thumbnails/two.png" Id="R2"/>
// after: keep only one
<Relationship Type="http://schemas.microsoft.com/xps/2005/06/thumbnail" Target="thumbnails/one.png" Id="R1"/>
Defensive patterns

Strategy: validation

Validate before calling

static int CountThumbnailRels(Package package)
{
    int n = 0;
    foreach (PackageRelationship rel in package.GetRelationshipsByType("http://schemas.microsoft.com/xps/2005/06/thumbnail")) n++;
    return n; // must be <= 1
}

Try / catch

try { var doc = XpsDocument.Open(...); }
catch (FileFormatException) { /* strip duplicate thumbnail rels and retry */ }

Prevention

When it happens

Trigger: Loading an XPS package where a part (typically the package root or FixedDocumentSequence/FixedDocument) declares more than one relationship of type http://schemas.microsoft.com/xps/2005/06/thumbnail.

Common situations: Packages merged from two XPS documents; packaging tools that append a second thumbnail relationship without removing the old one; programmatic package construction adding a thumbnail twice.

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/76b17e5be0469ccf. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedSchema.cs:643

                    Uri absTargetUri = PackUriHelper.Create(packageUri, targetUri);

                    PackagePart targetPart = package.GetPart(targetUri);

                    if (!_discardControlContentType.AreTypeAndSubTypeEqual(new ContentType(targetPart.ContentType)))
                    {
                        throw new FileFormatException(SR.XpsValidatingLoaderDiscardControlHasIncorrectType);
                    }
                }

                // This is the XPS payload root part. We also should check if the Package only has at most one thumbnail...
                checkRels = package.GetRelationshipsByType(_thumbnailRel);
                count = 0;
                foreach (PackageRelationship rel in checkRels)
                {
                    count++;
                    if (count > 1)
                    {
                        throw new FileFormatException(SR.XpsValidatingLoaderMoreThanOneThumbnailInPackage);
                    }

                    // Also check for existence and type
                    Uri targetUri = PackUriHelper.ResolvePartUri(partUri, rel.TargetUri);
                    Uri absTargetUri = PackUriHelper.Create(packageUri, targetUri);

                    PackagePart targetPart = package.GetPart(targetUri);

                    if (!_jpgContentType.AreTypeAndSubTypeEqual(new ContentType(targetPart.ContentType)) &&
                        !_pngContentType.AreTypeAndSubTypeEqual(new ContentType(targetPart.ContentType)))
                    {
                        throw new FileFormatException(SR.XpsValidatingLoaderThumbnailHasIncorrectType);
                    }
                }
            }
        }

        private

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