dotnet/maui · warning · NotSupportedException

Element IList<Element>.this[int index] { get => _inner[index

Error message

Element IList<Element>.this[int index] { get => _inner[index]; set => throw new NotSupportedException(); }

What it means

Thrown by the indexer setter of TemporaryWrapper, a private read-only IList<Element> adapter wrapping an IReadOnlyList<Element>. TemporaryWrapper is used by the obsolete Element.LogicalChildren property (for Hot Reload) to expose logical children as a ReadOnlyCollection. The setter throws NotSupportedException because the underlying list is read-only and the wrapper marks IsReadOnly = true.

Source

Thrown at src/Controls/src/Core/Element/Element.cs:1195

			else if (this is DatePicker dp && DateTime.TryParse(text, out DateTime dpResult))
			{
				dp.Date = dpResult;
				return true;
			}

			return false;
		}

		class TemporaryWrapper : IList<Element>
		{
			IReadOnlyList<Element> _inner;

			public TemporaryWrapper(IReadOnlyList<Element> inner)
			{
				_inner = inner;
			}

			Element IList<Element>.this[int index] { get => _inner[index]; set => throw new NotSupportedException(); }

			int ICollection<Element>.Count => _inner.Count;

			bool ICollection<Element>.IsReadOnly => true;

			void ICollection<Element>.Add(Element item) => throw new NotSupportedException();

			void ICollection<Element>.Clear() => throw new NotSupportedException();

			bool ICollection<Element>.Contains(Element item) => _inner.IndexOf(item) != -1;

			void ICollection<Element>.CopyTo(Element[] array, int arrayIndex) => throw new NotSupportedException();

			IEnumerator<Element> IEnumerable<Element>.GetEnumerator() => _inner.GetEnumerator();

			System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => _inner.GetEnumerator();

			int IList<Element>.IndexOf(Element item) => _inner.IndexOf(item);

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Use the public API: InsertLogicalChild / RemoveLogicalChild to modify logical children.
2. If consuming LogicalChildren, treat it as read-only (it is wrapped in ReadOnlyCollection by the public property).
3. Avoid reflecting into the internal TemporaryWrapper; it is explicitly a read-only adapter.

Example fix

// before (via reflection on internal wrapper)
inner[index] = newElement; // NotSupportedException
// after
element.InsertLogicalChild(index, newElement);

// LogicalChildren is read-only; never set the indexer. Use InsertLogicalChild.
if (element.LogicalChildren.IsReadOnly)
    return; // do not attempt writes

// This is a read-only adapter reached only via reflection; callers should not cast to a writable IList.

When it happens

Trigger: Casting the obsolete LogicalChildren ReadOnlyCollection's inner list and calling the indexer setter: element.LogicalChildren (via reflection or framework internals)[index] = newElement. The public ReadOnlyCollection already blocks this, so this only fires through reflection/internal access to the wrapper.

Common situations: Reflection-based tooling, serializers, or Hot Reload infrastructure attempting to write through the indexer of the LogicalChildren backing wrapper; incorrectly assuming the returned collection is mutable.

Related errors

• void ICollection<Element>.Add(Element item) => throw new Not
• void ICollection<Element>.Clear() => throw new NotSupportedE
• void ICollection<Element>.CopyTo(Element[] array, int arrayI
• void IList<Element>.Insert(int index, Element item) => throw
• bool ICollection<Element>.Remove(Element item) => throw new

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/60f10e07eaf4eddd. Report an issue: GitHub.