dotnet/wpf · error · COMException

FILTER_E_UNKNOWNFORMAT

FILTER_E_UNKNOWNFORMAT

Error message

File to filter is not loaded.

What it means

When the bytes are readable but not a valid XPS/OPC package, Package.Open throws FileFormatException, which Load translates into a COMException with hresult FILTER_E_UNKNOWNFORMAT. "File to filter is not loaded." again reflects that _filter stayed null in the finally block.

Solutions

1. Verify the file is a genuine XPS (OPC) package, e.g. opens in an XPS viewer
2. Fix file extension/source so the real format matches expectations
3. Regenerate or re-download a corrupt/partial file
4. Catch COMException with FILTER_E_UNKNOWNFORMAT and skip the file in indexing pipelines

Example fix

// before
filter.Load(path, (int)STGM_FLAGS.READ); // may hit non-XPS data
// after
try { filter.Load(path, (int)STGM_FLAGS.READ); }
catch (COMException ex) when (ex.HResult == unchecked((int)FilterErrorCode.FILTER_E_UNKNOWNFORMAT))
{
    // log and skip: not an XPS package
}

using (var fs = File.OpenRead(path))
using (var pk = new System.IO.Packaging.ZipPackage(fs)) { /* throws FileFormatException if not OPC */ }

try { filter.Load(path, (int)STGM_FLAGS.READ); }
catch (COMException ex) when (ex.HResult == unchecked((int)FilterErrorCode.FILTER_E_UNKNOWNFORMAT))
{ /* skip: not an XPS package */ }

When it happens

Trigger: Load called on a file whose stream is not an OPC/XPS container — wrong extension, ODF/DOCX/PDF renamed to .xps, or a corrupt/partial download.

Common situations: Indexers picking up files saved with wrong extensions; interrupted uploads stored as .xps; hand-crafted ZIPs missing [Content_Types].xml.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

• ' ' cannot contain the path delimiter: ' '.
• ' ' cannot start with the reserved character range…
• ' ' ID is not a valid XSD ID.
• 0x80004002
• 0x80040200

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/2054341a7b4840e4. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/IO/Packaging/XpsFilter.cs:427

                {
                    // Open the encrypted package.
                    _encryptedPackage = EncryptedPackageEnvelope.Open(_packageStream);
                    _filter = new EncryptedPackageFilter(_encryptedPackage);
                }
                else
                {
                    // Open the package.
                    _package = Package.Open(_packageStream);
                    _filter = new PackageFilter(_package);
                }
            }
            catch (IOException ex)
            {
                throw new COMException(ex.Message, (int)FilterErrorCode.FILTER_E_ACCESS);
            }
            catch (FileFormatException ex)
            {
                throw new COMException(ex.Message, (int)FilterErrorCode.FILTER_E_UNKNOWNFORMAT);
            }
            finally
            {
                // failure?
                if (_filter == null)
                {
                    // clean up
                    ReleaseResources();
                }
}

            _xpsFileName = pszFileName;
        }

        /// <summary>
        /// Saves a copy of the object into the specified file.
        /// </summary>
        /// <param name="pszFileName">

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