What causes "SR.MustBeOfType" in dotnet/wpf?

Pass an object implementing IEnumerable (enumerating PageContent) to the async collection serializer

dotnet/wpf · error · XpsSerializationException

SR.MustBeOfType

Error message

SR.MustBeOfType

What it means

Async variant of the same guard: ReachPageContentCollectionSerializerAsync.PersistObjectData throws this XpsSerializationException when the context's TargetObject is not IEnumerable. The asynchronous page-content collection serializer can only enumerate collections of PageContent items.

Solutions

Pass an object implementing IEnumerable (enumerating PageContent) to the async collection serializer
Use ReachPageContentSerializerAsync for individual page content items
Inspect the SerializableObjectContext.TargetObject type before the async operation is queued
Keep the serializer registration consistent with the object types in the document tree

Example fix

// before
context = new SerializableObjectContext(fixedPage, "Pages"); // non-enumerable target
// after
context = new SerializableObjectContext(fixedDocument.Pages /* IEnumerable */, "Pages");

Defensive patterns

Strategy: validation

Validate before calling

if (ctx.TargetObject is not IEnumerable) throw new ArgumentException("Async collection serializer requires IEnumerable target");

Type guard

bool IsEnumerableTarget(SerializableObjectContext c) => c?.TargetObject is IEnumerable;

Prevention

Validate context targets before pushing async operations
Mirror sync/async target types in custom pipelines
Test async save paths with representative document trees

When it happens

Trigger: Async XPS serialization where the SerializableObjectContext handed to PersistObjectData wraps a non-collection object (single PageContent, FixedPage, or unrelated type) instead of an enumerable PageContent collection.

Common situations: Same as the sync case but in async save paths (XpsSerializationManagerAsync): mis-wired context construction, custom serializers registered for the wrong types, refactors that changed the target's type away from a collection.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/79e5571983f26a52. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Serialization/manager/ReachPageContentCollectionSerializerAsync.cs:111

        /// point of the serialization process.
        /// </summary>
        /// <param name="serializableObjectContext">
        /// The context of the object to be serialized at this time.
        /// </param>
        internal
        override
        void
        PersistObjectData(
            SerializableObjectContext   serializableObjectContext
            )
        {
            ArgumentNullException.ThrowIfNull(serializableObjectContext);

            IEnumerable enumerableObject = serializableObjectContext.TargetObject as IEnumerable;

            if (enumerableObject == null)
            {
                throw new XpsSerializationException(SR.Format(SR.MustBeOfType, "serializableObjectContext.TargetObject", typeof(IEnumerable)));
            }

            //
            // Serialize the PageContent Items contained within the collection 
            //
            SerializePageContents(serializableObjectContext);
        }

        internal
        override
        void
        EndPersistObjectData(
            )
        {
            //
            // do nothing in this stage
            //
        }

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