dotnet/wpf · error · XpsSerializationException

SR.Format(SR.MustBeOfType…

Error message

SR.Format(SR.MustBeOfType, "serializableObjectContext.TargetObject", typeof(IEnumerable))

What it means

The PageContent collection serializer requires the serializable object context's TargetObject to implement IEnumerable; it throws XpsSerializationException(SR.Format(SR.MustBeOfType, "serializableObjectContext.TargetObject", typeof(IEnumerable))) otherwise. It enumerates PageContent items inside a FixedDocument, so a non-enumerable target is an invariant violation.

Solutions

Ensure the context TargetObject is the FixedDocument's Pages/PageContent collection (IEnumerable), not a single page.
Fix custom context construction so it passes the enumerable collection property.
Validate with 'is IEnumerable' before creating the context.

Example fix

// before
new NgcSerializableObjectContext(pageContent, ""); // single item
// after
new NgcSerializableObjectContext(fixedDocument.Pages, ""); // IEnumerable collection

Defensive patterns

Strategy: validation

Validate before calling

if (serializableObjectContext?.TargetObject is not IEnumerable)
    throw new InvalidOperationException("PageContent serializer context target must be an IEnumerable collection");

Type guard

bool HasEnumerableTarget(NgcSerializableObjectContext ctx) => ctx?.TargetObject is IEnumerable;

Try / catch

try { SerializePageContents(context); }
catch (XpsSerializationException ex) when (ex.Message.Contains("IEnumerable")) { /* rebuild the context with the collection property */ }

Prevention

Always build serializer contexts from collection properties (e.g. FixedDocument.Pages)
Never pass a single PageContent as a collection context target
Assert context target types in serializer unit tests

When it happens

Trigger: Serializing a FixedDocument's PageContents where the context's TargetObject is not an IEnumerable (e.g. a single PageContent or a wrong property was pushed as the context target).

Common situations: Custom serializer contexts built by hand with the wrong TargetObject; internal context wiring broken after overriding serialization steps.

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/94041e8177908bfe. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Serialization/manager/NGCSerializer.cs:676

        #endregion Constructor


        /// <summary>
        ///
        /// </summary>
        public
        override
        void
        SerializeObject(
            Object serializedObject
            )
        {
            IEnumerable enumerableObject =  serializedObject 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(enumerableObject);
        }

        #region Private Methods

        /// <summary>
        /// This is being called to serialize the Page Content items
        /// contained within the collection
        /// </summary>
        /// <param name="enumerableObject">
        /// The context of the object to be serialized at this time.
        /// </param>
        private

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