dotnet/wpf · error · FileFormatException

SR.CertificatePartContentTypeMismatch

Error message

SR.CertificatePartContentTypeMismatch

What it means

The CertificatePart constructor throws this FileFormatException when the existing package part at partName has a content type that does not match the expected certificate-part content type. The package contains a part at the certificate location that is not a certificate part.

Solutions

1. Ensure the certificate part's content type is the standard package certificate content type
2. Re-save the package so the certificate part is stored with the correct content type
3. Catch the mismatch exception when opening untrusted packages

When it happens

Trigger: Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/IO/Packaging/Certificate.cs:145 when the library encounters an invalid state.

Common situations: See trigger scenarios.

Understand the failure class

• SSL/TLS and certificate errors — how TLS handshakes and certificate validation fail.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/63fa8bedb404550b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/IO/Packaging/Certificate.cs:145

        /// <summary>
        /// CertificatePart constructor
        /// </summary>
        internal CertificatePart(System.IO.Packaging.Package container, Uri partName)
        {
            ArgumentNullException.ThrowIfNull(container);
            ArgumentNullException.ThrowIfNull(partName);

            partName = PackUriHelper.ValidatePartUri(partName);
            
            // create if not found
            if (container.PartExists(partName))
            {
                // open the part
                _part = container.GetPart(partName);

                // ensure the part is of the expected type
                if (!_part.ValidatedContentType().AreTypeAndSubTypeEqual(_certificatePartContentType))
                    throw new FileFormatException(SR.CertificatePartContentTypeMismatch);
            }
            else
            {
                // create the part
                _part = container.CreatePart(partName, _certificatePartContentType.ToString());
            }
        }

        //------------------------------------------------------
        //
        //  Private Fields
        //
        //------------------------------------------------------
        private PackagePart             _part;          // part that houses the certificate
        private X509Certificate2       _certificate;   // certificate itself

        // certificate part constants
        private static readonly ContentType _certificatePartContentType =

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