dotnet/wpf · error · XmlException

SR.SignatureObjectIdMustBeUnique

Error message

SR.SignatureObjectIdMustBeUnique

What it means

GetPackageDataObject scans the signature's ObjectList for the DataObject whose Id is the OPC package object id. Two DataObjects with the same Id make the selection ambiguous, so XmlException(SR.SignatureObjectIdMustBeUnique) is thrown: per XML DSig, Object element Id attributes must be unique within a signature.

Solutions

  1. Re-sign the package with PackageDigitalSignatureManager.Sign so a single, unique package Object is generated.
  2. If adding custom Objects, assign each a distinct Id attribute different from the OPC package object Id.
  3. Inspect the signature XML and remove the duplicate <Object> element with the reserved Id.
  4. Treat the signature as corrupt and remove it (dsm.RemoveSignature(sig)) if it cannot be repaired.

Example fix

// before: duplicate Ids
<Object Id="idPackageObject">...</Object>
<Object Id="idPackageObject">...</Object>
// after: unique Ids
<Object Id="idPackageObject">...</Object>
<Object Id="myCustomObject">...</Object>
Defensive patterns

Strategy: validation

Validate before calling

var ids = doc.SelectNodes("//*[local-name()='Object']/@Id")
              .Cast<XmlAttribute>().Select(a => a.Value).ToList();
bool unique = ids.Count == ids.Distinct().Count();

Type guard

static bool ObjectIdsUnique(XmlDocument doc) =>
    doc.SelectNodes("//*[local-name()='Object']/@Id").Cast<XmlAttribute>()
       .GroupBy(a => a.Value).All(g => g.Count() == 1);

Try / catch

try { dsm.VerifySignatures(true); }
catch (XmlException ex) { /* duplicate Object Id — remove and re-sign */ }

Prevention

When it happens

Trigger: Verify or property access on a PackageSignature whose SignedXml Signature.ObjectList contains two or more <Object> elements sharing the package object Id (the OPC-reserved id), e.g. after custom code appended a second Object with the same Id.

Common situations: Custom signing code that adds its own Object without changing the Id; repeated application of a signing step that appends instead of replaces; merging signatures from two documents.

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/08a2ddb2a4861f93. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/IO/Packaging/XmlDigitalSignatureProcessor.cs:1049

        /// <summary>
        /// Finds and return the package-specific Object tag
        /// </summary>
        /// <returns></returns>
        private DataObject GetPackageDataObject()
        {
            EnsureXmlSignatureParsed();

            // look for the Package-specific object tag
            String opcId = XTable.Get(XTable.ID.OpcAttrValue);
            DataObject returnValue = null;
            foreach (DataObject dataObject in _signedXml.Signature.ObjectList)
            {
                if (string.Equals(dataObject.Id, opcId, StringComparison.Ordinal))
                {
                    // duplicates not allowed
                    if (returnValue != null)
                        throw new XmlException(SR.SignatureObjectIdMustBeUnique);

                    returnValue = dataObject;
                }
            }

            // Package object tag required
            if (returnValue != null)
                return returnValue;
            else
                throw new XmlException(SR.PackageSignatureObjectTagRequired);
        }

        private KeyInfo GenerateKeyInfo(AsymmetricAlgorithm key, X509Certificate2 signer)
        {
            // KeyInfo section
            KeyInfo keyInfo = new KeyInfo();
            KeyInfoName keyInfoName = new KeyInfoName
            {

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