dotnet/wpf · error · NotSupportedException

SR.OnlySupportedOnCollections

Error message

SR.OnlySupportedOnCollections

What it means

XamlTypeInvoker.AddToCollection only works on XamlTypes whose IsCollection is true (a type with a usable Add method or implementing IList). Calling it on any other type throws NotSupportedException with SR.OnlySupportedOnCollections.

Solutions

  1. Check `_xamlType.IsCollection` (or the member's type IsCollection) before calling AddToCollection and branch accordingly.
  2. For dictionaries use XamlTypeInvoker.AddToDictionary instead.
  3. Fix the type mapping so the target really is a collection type, or wrap the value in a collection first.

Example fix

// before
invoker.AddToCollection(instance, item); // NotSupportedException for non-collection
// after
if (typeInvoker.ShouldSerializeCollection && xamlType.IsCollection)
    typeInvoker.AddToCollection(instance, item);
else if (xamlType.IsDictionary)
    typeInvoker.AddToDictionary(instance, key, item);
Defensive patterns

Strategy: type-guard

Validate before calling

bool isCollection = xamlType.IsCollection; // also consider IsDictionary

Type guard

static bool CanAddToCollection(XamlType t) => t.IsCollection;

Try / catch

try { invoker.AddToCollection(instance, item); }
catch (NotSupportedException) { /* not a collection: use AddToDictionary or skip */ }

Prevention

When it happens

Trigger: Calling invoker.AddToCollection(instance, item) where the instance's XamlType is not a collection — e.g. appending to a plain POCO, a dictionary (use AddToDictionary instead), or after constructing an invoker for the wrong type.

Common situations: Generic XAML item-collection code that treats every object member as a collection; mismapping where a type's collection nature changed between library versions (e.g. property changed from List to single value); unit tests exercising AddToCollection on non-collection types.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a07f4f678de443e5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XamlTypeInvoker.cs:69

        public EventHandler<XamlSetTypeConverterEventArgs> SetTypeConverterHandler
        {
            get { return _xamlType?.SetTypeConverterHandler; }
        }

        public virtual void AddToCollection(object instance, object item)
        {
            ArgumentNullException.ThrowIfNull(instance);
            if (instance is IList list)
            {
                list.Add(item);
                return;
            }

            ThrowIfUnknown();
            if (!_xamlType.IsCollection)
            {
                throw new NotSupportedException(SR.OnlySupportedOnCollections);
            }

            XamlType itemType;
            if (item is not null)
            {
                itemType = _xamlType.SchemaContext.GetXamlType(item.GetType());
            }
            else
            {
                itemType = _xamlType.ItemType;
            }

            MethodInfo addMethod = GetAddMethod(itemType);
            if (addMethod is null)
            {
                throw new XamlSchemaException(SR.Format(SR.NoAddMethodFound, _xamlType, itemType));
            }

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