dotnet/wpf · error · XpsSerializationException

SR.MustBeOfType

Error message

SR.MustBeOfType

What it means

ReachUIElementCollectionSerializer.PersistObjectData only knows how to serialize objects implementing IEnumerable, because it enumerates the target to serialize each contained UI element. If the context's TargetObject is not enumerable, the serializer was matched to the wrong object type and an XpsSerializationException naming IEnumerable is thrown.

Solutions

  1. Make the container implement IEnumerable (and ideally ICollection/UIElementCollection semantics) so it is serializable
  2. Use standard WPF collections (UIElementCollection, VisualCollection) in serialized trees
  3. Ensure the serializer selection maps the object to the correct serializer type rather than the collection serializer

Example fix

// before
class MyContainer : Visual { public List<Visual> Items = new List<Visual>(); }
// after
class MyContainer : Visual, IEnumerable { public List<Visual> Items = new List<Visual>(); public IEnumerator GetEnumerator() => Items.GetEnumerator(); }
Defensive patterns

Strategy: type-guard

Validate before calling

if (obj is not IEnumerable) throw new ArgumentException("Target must implement IEnumerable to use the UI element collection serializer");

Type guard

bool IsEnumerableCollection(object o) => o is IEnumerable;

Try / catch

try { serializer.SerializeObject(collection); }
catch (XpsSerializationException ex) when (ex.Message.Contains("IEnumerable")) { /* use a different serializer or fix the container type */ }

Prevention

When it happens

Trigger: Serializing an object whose type resolved to ReachUIElementCollectionSerializer (e.g. a UIElementCollection-like container) but whose runtime TargetObject does not implement IEnumerable — typically a custom collection or container type that mimics the serialized shape without implementing IEnumerable.

Common situations: Custom panels/collections substituted for standard WPF collections in a serialized visual tree; type-substitution bugs where a serializer is selected for a non-enumerable object.

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/0392385d5b2ba41b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Serialization/manager/ReachUIElementCollectionSerializer.cs:88

        /// 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

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