dotnet/wpf · error · InvalidOperationException

Cannot remove signature from read-only file.

Error message

Cannot remove signature from read-only file.

What it means

PackageDigitalSignatureManager.RemoveSignature(Uri) refuses to modify a package opened in read-only mode. The manager exposes ReadOnly and throws InvalidOperationException before touching the signatures, since a read-only Package cannot have parts or relationships deleted. Caller should call Package.Flush() afterward to persist successful removals.

Solutions

1. Reopen the package with FileAccess.ReadWrite (e.g. Package.Open(path, FileMode.Open, FileAccess.ReadWrite)) before calling RemoveSignature.
2. Check PackageDigitalSignatureManager.ReadOnly before calling and surface a clear message or copy the package to a writable location first.
3. If the source file itself is locked or on read-only media, copy it locally, clear the read-only attribute, and open read-write.

Example fix

// before
using Package pkg = Package.Open(path, FileMode.Open, FileAccess.Read);
var mgr = new PackageDigitalSignatureManager(pkg);
mgr.RemoveSignature(sigUri); // InvalidOperationException
// after
using Package pkg = Package.Open(path, FileMode.Open, FileAccess.ReadWrite);
var mgr = new PackageDigitalSignatureManager(pkg);
mgr.RemoveSignature(sigUri);
pkg.Flush();

if (mgr.ReadOnly) throw new InvalidOperationException("Package is read-only; reopen with FileAccess.ReadWrite before removing signatures.");
mgr.RemoveSignature(signatureUri);

bool CanModify(PackageDigitalSignatureManager mgr) => !mgr.ReadOnly;

try { mgr.RemoveSignature(uri); pkg.Flush(); }
catch (InvalidOperationException ex) { /* read-only package: reopen writable or report */ }

When it happens

Trigger: Calling RemoveSignature(uri) on a manager whose underlying Package was opened with FileAccess.Read or FileMode.Open + FileAccess.Read, leaving ReadOnly == true.

Common situations: Opening a .oxps/.docx-like OPC package from a read-only share, from a stream the caller only has read access to, or with Package.Open(path, FileMode.Open, FileAccess.Read) and then trying to edit signatures.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• ' ' ID is not a valid XSD ID.
• Cannot sign read-only file.
• Image_OriginalStreamReadOnly
• NotSupportedException
• Object identifiers must be unique within the same signature.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/8f8856a718b47623. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/IO/Packaging/PackageDigitalSignatureManager.cs:687

                        if (exitOnFailure)
                            break;
                    }
                }
            }

            return result;
        }

        /// <summary>
        /// Remove a signature
        /// </summary>
        /// <param name="signatureUri">signature to remove</param>
        /// <remarks>Caller should call Package.Flush() in order to persist changes.</remarks>
        public void RemoveSignature(Uri signatureUri)
        {
            if (ReadOnly)
                throw new InvalidOperationException(SR.CannotRemoveSignatureFromReadOnlyFile);

            ArgumentNullException.ThrowIfNull(signatureUri);

            // empty?
            if (!IsSigned)      // calls EnsureSignatures for us
                return;

            // find the signature
            int index = GetSignatureIndex(signatureUri);
            if (index < 0)
                return;

            try
            {
                Debug.Assert(index < _signatures.Count);

                //After this signature is removed the total number of signatures remaining will
                //be _signatures.Count - 1. If this count is zero, then additional clean up needs

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