dotnet/wpf · error · XpsSerializationException

SR.Format(SR.MustBeOfType…

Error message

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

What it means

XpsSerializationException thrown by NGCUIElementCollectionSerializerAsync.PersistObjectData when the serialization context's TargetObject cannot be cast to IEnumerable. This serializer is only designed to serialize collections (specifically collections of page/UI elements) during XPS document writing, so it rejects any object it was handed that is not enumerable.

Solutions

Ensure the object passed to the serializer's PersistObjectData has a TargetObject that implements System.Collections.IEnumerable (e.g. a UIElementCollection).
If serializing a single element, wrap it in a collection or route it to the appropriate single-object serializer instead of NGCUIElementCollectionSerializerAsync.
Verify custom serializer registration/mapping does not bind this collection serializer to non-collection types.
Debug serializableObjectContext.TargetObject.GetType() at the throw site to identify which non-enumerable type leaked in.

Example fix

// before
serializerContext.TargetObject = mySingleVisual; // not IEnumerable
// after
serializerContext.TargetObject = myVisualCollection; // implements IEnumerable

Defensive patterns

Strategy: validation

Validate before calling

if (context?.TargetObject is not System.Collections.IEnumerable)
    throw new ArgumentException("TargetObject must implement IEnumerable", nameof(context));

Type guard

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

Try / catch

try { serializer.PersistObjectData(context, ...); }
catch (XpsSerializationException ex) when (ex.Message.Contains("IEnumerable"))
{ /* log type of context.TargetObject and fall back to single-object serializer */ }

Prevention

Always hand collection serializers an object implementing IEnumerable.
Unit-test serializer registration mappings against the types they will receive.
Log TargetObject.GetType() before serialization in debug builds.
Wrap single elements in a containing collection when a collection serializer is required.

When it happens

Trigger: Calling XpsDocument/XpsSerializationManager APIs with a SerializableObjectContext whose TargetObject is a non-IEnumerable object (e.g. a single UIElement or arbitrary object) instead of a collection such as UIElementCollection or a FixedDocument/PageContent collection.

Common situations: Passing the wrong object to a custom serializer or document sequence during XPS export; registering this serializer for a type it was not designed for; refactoring printing code so a single element instead of its parent collection reaches the serializer.

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/deded941f454e84e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Serialization/manager/NGCUIElementCollectionSerializerAsync.cs:127

        /// 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 
            //
            SerializeUIElements(serializableObjectContext);
        }

        #endregion Internal Methods


        #region Private Methods

        /// <summary>
        /// This is being called to serialize the Page Content items
        /// contained within the collection
        /// </summary>
        private

View on GitHub (pinned to 81131a70a4)