unoplatform/uno · error · NotSupportedException

Non-collection type '{0}' does not support this operation

Error message

Non-collection type '{0}' does not support this operation

What it means

Thrown by XamlTypeInvoker.AddToCollection when the instance's XamlType is known (UnderlyingType non-null) but IsCollection is false. The operation only makes sense on collections; a non-collection type has no Add semantics. This early check rejects the operation before attempting reflection.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeInvoker.cs:85

		public EventHandler<XamlSetTypeConverterEventArgs> SetTypeConverterHandler {
			get { return type == null ? null : type.SetTypeConverterHandler; }
		}

		public virtual void AddToCollection (object instance, object item)
		{
			if (instance == null)
				throw new ArgumentNullException ("instance");
			if (item == null)
				throw new ArgumentNullException ("item");

			var ct = instance.GetType ();
			var xct = type == null ? null : type.SchemaContext.GetXamlType (ct);
			MethodInfo mi = null;

			// FIXME: this method lookup should be mostly based on GetAddMethod(). At least iface method lookup must be done there.
			if (type != null && type.UnderlyingType != null) {
				if (!xct.IsCollection) // not sure why this check is done only when UnderlyingType exists...
					throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Non-collection type '{0}' does not support this operation", xct));
				if (ct.IsAssignableFrom (type.UnderlyingType))
					mi = GetAddMethod (type.SchemaContext.GetXamlType (item.GetType ()));
			}

			if (mi == null) {
				if (ct.IsGenericType) {
					mi = ct.GetMethod ("Add", ct.GetGenericArguments ());
					if (mi == null)
						mi = LookupAddMethod (ct, typeof (ICollection<>).MakeGenericType (ct.GetGenericArguments ()));
				} else {
					mi = ct.GetMethod ("Add", new Type [] {typeof (object)});
					if (mi == null)
						mi = LookupAddMethod (ct, typeof (IList));
				}
			}

			if (mi == null)
				throw new InvalidOperationException (String.Format (CultureInfo.InvariantCulture, "The collection type '{0}' does not have 'Add' method", ct));

View on GitHub (pinned to 0418340488)

Solutions

1. Verify the target type implements IEnumerable/IList/Add before calling AddToCollection — check xamlType.IsCollection / LookupCollectionKind().
2. Correct the XAML: ensure the property/element is actually a collection type, or fix the ContentProperty attribute.
3. Fix the XamlSchemaContext type mapping so the right (collection) XamlType is resolved.

Example fix

// before
type.Invoker.AddToCollection(instance, item);

// after
if (!type.IsCollection) {
    throw new InvalidOperationException($"{type} is not a collection; cannot add items.");
}
type.Invoker.AddToCollection(instance, item);

// before AddToCollection
if (!type.IsCollection) {
    throw new InvalidOperationException($"{type} is not a collection; cannot add items.");
}

When it happens

Trigger: Calling AddToCollection on an instance whose XamlType is not flagged as a collection — e.g. a scalar object, a class without IEnumerable/IList/Add, or a type whose CollectionKind resolved to None.

Common situations: XAML where an element is treated as a content collection but the target type is not a collection (mismatched ContentProperty, wrong type mapping); custom writers that assume a property holds a collection when it holds a single object; type-resolution returning a different (non-collection) XamlType than expected.

Related errors

• type
• Current operation is valid only when the underlying type on
• instance
• item
• The collection type '{0}' does not have 'Add' method

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/e119c7426b85e436. Report an issue: GitHub.