dotnet/maui · warning · NotSupportedException

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

Error message

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

What it means

Thrown by TemporaryWrapper.Clear(). TemporaryWrapper is the read-only IList<Element> adapter behind the obsolete Element.LogicalChildren ReadOnlyCollection. Because IsReadOnly is true and the inner IReadOnlyList cannot be cleared, Clear throws NotSupportedException.

Source

Thrown at src/Controls/src/Core/Element/Element.cs:1203

		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);

			void IList<Element>.Insert(int index, Element item) => throw new NotSupportedException();

			bool ICollection<Element>.Remove(Element item) => throw new NotSupportedException();

			void IList<Element>.RemoveAt(int index) => throw new NotSupportedException();
		}
	}

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Remove logical children individually via RemoveLogicalChild rather than clearing the collection.
2. Treat LogicalChildren and its backing list as read-only.
3. Avoid writing to internal Element collections.

Example fix

// before (reflection on internal wrapper)
wrapper.Clear(); // NotSupportedException
// after
foreach (var child in element.LogicalChildren.ToList())
    element.RemoveLogicalChild(child);

if (element.LogicalChildren.IsReadOnly)
    return; // do not Clear the backing wrapper

// Adapter reached only via reflection; remove children individually via RemoveLogicalChild.

When it happens

Trigger: Calling Clear on the internal TemporaryWrapper obtained via reflection or internal access. The public ReadOnlyCollection wrapper would block this first; reaching TemporaryWrapper.Clear requires accessing the adapter directly.

Common situations: Hot Reload or designer tooling attempting to reset logical children by clearing the backing collection; reflection-based code treating the wrapper as mutable.

Related errors

• Element IList<Element>.this[int index] { get => _inner[index
• void ICollection<Element>.Add(Element item) => throw new Not
• 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/661c29ceb0ef548f. Report an issue: GitHub.