dotnet/wpf · error · XpsSerializationException

SR.Format(SR.MustBeOfType…

Error message

SR.Format(SR.MustBeOfType, "serializableObjectContext.TargetObject", typeof(System.Collections.Generic.IEnumerable<DocumentReference>))

What it means

ReachDocumentReferenceCollectionSerializer.PersistObjectData throws this XpsSerializationException when the serializer's target object is not an IEnumerable<DocumentReference>. The serializer is registered for DocumentReference collections; if the object context handed to it cannot be cast (the 'as' returns null), the input type doesn't match the serializer's contract, so serialization aborts with a MustBeOfType message naming the expected type.

Solutions

Ensure the object serialized at this point is (or implements) IEnumerable<DocumentReference>; inspect what type is being passed to SerializeObject.
Fix the serializer registration/type mapping so ReachDocumentReferenceCollectionSerializer is only used for DocumentReference collections.
Cast or wrap the data into IEnumerable<DocumentReference> before handing it to the serialization manager.

Example fix

// before: wrong type handed to serializer
serializer.SerializeObject(fixedDocumentSequence);

// after: serialize the DocumentReferences collection
IEnumerable<DocumentReference> refs = fixedDocumentSequence.References;
serializer.SerializeObject(refs);

Defensive patterns

Strategy: type-guard

Validate before calling

if (target is not IEnumerable<DocumentReference>)
    throw new ArgumentException(nameof(target), "Serializer expects IEnumerable<DocumentReference>");

Type guard

bool IsDocRefCollection(object o) => o is IEnumerable<DocumentReference>;

Try / catch

try { serializer.SerializeObject(obj); }
catch (XpsSerializationException ex) when (ex.Message.Contains("MustBeOfType")) {
    // wrong serializer for this object type; fix registration
}

Prevention

Register serializers only for their documented contract types
Unit-test serializer type dispatch with representative objects
Validate target object type before serialization

When it happens

Trigger: Registering or invoking ReachDocumentReferenceCollectionSerializer for an object whose TargetObject is not IEnumerable<DocumentReference> — e.g., a single DocumentReference, a FixedDocumentSequence, or a custom collection of a different element type passed through XpsSerializationManager.

Common situations: Custom serializer registration tables mapping the wrong type to this serializer; passing a DocumentSequence or package-level object to the collection serializer by mistake; refactors that changed the collection element type so the 'as' cast fails at runtime.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/71ed79ffec6cba4f. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Serialization/manager/ReachDocumentReferenceCollectionSerializer.cs:45

        /// <summary>
        /// 
        /// </summary>
        internal
        override
        void
        PersistObjectData(
            SerializableObjectContext   serializableObjectContext
            )
        {
            ArgumentNullException.ThrowIfNull(serializableObjectContext);

            // get DocumentReferenceCollection
            System.Collections.Generic.IEnumerable<DocumentReference> enumerableObject = serializableObjectContext.TargetObject as System.Collections.Generic.IEnumerable<DocumentReference>;

            if (enumerableObject == null)
            {
                throw new XpsSerializationException(SR.Format(SR.MustBeOfType, "serializableObjectContext.TargetObject", typeof(System.Collections.Generic.IEnumerable<DocumentReference>)));
            }

            SerializeDocumentReferences(serializableObjectContext);
        }

        /// <summary>
        /// This is being called to serialize the DocumentReference items
        /// contained within the colleciton
        /// </summary>
        private
        void
        SerializeDocumentReferences(
            SerializableObjectContext   serializableObjectContext
            )
        {
            //
            // Serialize each DocumentReference in DocumentReferenceColleciton
            //

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