AvaloniaUI/Avalonia · error · NotSupportedException

Unsupported AvaloniaProperty type.

Error message

Unsupported AvaloniaProperty type.

What it means

Thrown by AvaloniaObject.ClearValue<T> when the supplied property is neither a StyledProperty<T> nor a DirectPropertyBase<T>. The generic ClearValue dispatches on the runtime property kind; any other AvaloniaProperty subclass (or an untyped/registered-as-other property) hits the default branch.

Source

Thrown at src/Avalonia.Base/AvaloniaObject.cs:157

        /// <summary>
        /// Clears a <see cref="AvaloniaProperty"/>'s local value.
        /// </summary>
        /// <param name="property">The property.</param>
        public void ClearValue<T>(AvaloniaProperty<T> property)
        {
            ThrowHelper.ThrowIfNull(property, nameof(property));
            VerifyAccess();

            switch (property)
            {
                case StyledProperty<T> styled:
                    ClearValue(styled);
                    break;
                case DirectPropertyBase<T> direct:
                    ClearValue(direct);
                    break;
                default:
                    throw new NotSupportedException("Unsupported AvaloniaProperty type.");
            }
        }

        /// <summary>
        /// Clears a <see cref="AvaloniaProperty"/>'s local value.
        /// </summary>
        /// <param name="property">The property.</param>
        public void ClearValue<T>(StyledProperty<T> property)
        {
            ThrowHelper.ThrowIfNull(property, nameof(property));
            VerifyAccess();

            _values.ClearValue(property);
        }

        /// <summary>
        /// Clears a <see cref="AvaloniaProperty"/>'s local value.
        /// </summary>

View on GitHub (pinned to 11c5427268)

Solutions

1. Use the property-type-specific overload: ClearValue(StyledProperty<T>) or ClearValue(DirectPropertyBase<T>).
2. Cast the property to its concrete type before clearing.
3. For attached properties, use the static ClearValue(provider, AttachedProperty) helper on the owning type.

Example fix

// before
obj.ClearValue((AvaloniaProperty<double>)someUnknownProp);

// after
if (someUnknownProp is StyledProperty<double> styled)
    obj.ClearValue(styled);
else if (someUnknownProp is DirectPropertyBase<double> direct)
    obj.ClearValue(direct);

switch (property)
{
    case StyledProperty<T> s: obj.ClearValue(s); break;
    case DirectPropertyBase<T> d: obj.ClearValue(d); break;
    default: throw new NotSupportedException();
}

static bool IsClearable(AvaloniaProperty<T> p) =>
    p is StyledProperty<T> or DirectPropertyBase<T>;

When it happens

Trigger: Calling ClearValue<T> with a property instance whose runtime type is a custom AvaloniaProperty subclass; passing an AttachedProperty that does not derive from StyledProperty<T>; calling the wrong generic arity.

Common situations: Reflection-based code that resolves an AvaloniaProperty by name and calls ClearValue generically; mixing styled and direct property APIs.

Related errors

• Binding returned unsupported {nameof(BindingExpressionBase)}
• The property {property.Name} is readonly.
• Unsupported AvaloniaProperty type.
• Invalid priority ${priority}
• 'name' may not contain periods.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/dfeda7f9e59dd903. Report an issue: GitHub.