unoplatform/uno · error · ArgumentNullException

instance

Error message

instance

What it means

Thrown by XamlTypeInvoker.AddToCollection when the `instance` argument is null. AddToCollection reflects an `Add` method on the instance and invokes it, so a null collection has no type to reflect and nothing to add to.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeInvoker.cs:74

		void ThrowIfUnknown ()
		{
			if (type == null || type.UnderlyingType == null)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Current operation is valid only when the underlying type on a XamlType is known, but it is unknown for '{0}'", type));
		}

		public EventHandler<XamlSetMarkupExtensionEventArgs> SetMarkupExtensionHandler {
			get { return type == null ? null : type.SetMarkupExtensionHandler; }
		}

		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 ());

View on GitHub (pinned to 0418340488)

Solutions

1. Ensure the collection instance is resolved and non-null before calling AddToCollection (e.g. instantiate the collection in the property getter if null).
2. In a custom object writer, check that the current object on the stack is non-null at the point of adding collection items.
3. Guard: `if (instance == null) { ...initialize or throw with context... }` before the call.

Example fix

// before
type.Invoker.AddToCollection(collection, item); // collection may be null

// after
collection ??= (IList)Activator.CreateInstance(collectionType);
type.Invoker.AddToCollection(collection, item);

// before AddToCollection
if (instance == null) {
    throw new InvalidOperationException("Collection instance is null; initialize it before adding items.");
}

When it happens

Trigger: Calling AddToCollection(null, item) directly, or a XAML object-writer path where the collection object (from GetItems or a property value) was null when processing a collection-member child item.

Common situations: XAML where a collection property is lazily initialized but the writer attempted to add children before the collection was created; custom writers that resolve the parent collection to null; null-propagation from a failed property get.

Related errors

• item
• type
• Non-collection type '{0}' does not support this operation
• The collection type '{0}' does not have 'Add' method
• Attempt to set value from read-only property {0}

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/6856d26deaef6c5d. Report an issue: GitHub.