AvaloniaUI/Avalonia · error · InvalidOperationException

The metadata is read-only.

Error message

The metadata is read-only.

What it means

Thrown by AvaloniaPropertyMetadata.Merge when the metadata instance has been frozen (IsReadOnly == true). Avalonia freezes property metadata once the property is fully registered so that shared metadata cannot be mutated after the fact. Calling Merge on already-frozen metadata violates that immutability contract.

Source

Thrown at src/Avalonia.Base/AvaloniaPropertyMetadata.cs:65

        public bool? EnableDataValidation { get; private set; }

        /// <summary>
        /// Gets whether this instance is read-only and can't be modified.
        /// </summary>
        public bool IsReadOnly { get; private set; }

        /// <summary>
        /// Merges the metadata with the base metadata.
        /// </summary>
        /// <param name="baseMetadata">The base metadata to merge.</param>
        /// <param name="property">The property to which the metadata is being applied.</param>
        public virtual void Merge(
            AvaloniaPropertyMetadata baseMetadata, 
            AvaloniaProperty property)
        {
            if (IsReadOnly)
            {
                throw new InvalidOperationException("The metadata is read-only.");
            }

            if (_defaultBindingMode == BindingMode.Default)
            {
                _defaultBindingMode = baseMetadata.DefaultBindingMode;
            }

            EnableDataValidation ??= baseMetadata.EnableDataValidation;
        }

        /// <summary>
        /// Makes this instance read-only.
        /// No further modifications are allowed after this call.
        /// </summary>
        public void Freeze()
            => IsReadOnly = true;

        /// <summary>

View on GitHub (pinned to 11c5427268)

Solutions

1. Ensure Merge/OverrideMetadata is called before the property registration freezes the metadata.
2. Call GenerateTypeSafeMetadata() to obtain a fresh, writable copy of the metadata and merge into that instead.
3. If you hold a reference to frozen metadata, create a new AvaloniaPropertyMetadata instance with the desired values rather than mutating the frozen one.
4. Check metadata.IsReadOnly before calling Merge and skip or copy accordingly.

Example fix

// before (mutates frozen metadata)
property.GetMetadata(ownerType).Merge(baseMetadata, property);

// after (work on a fresh copy)
if (metadata.IsReadOnly) {
    var copy = metadata.GenerateTypeSafeMetadata();
    copy.Merge(baseMetadata, property);
}

if (metadata.IsReadOnly) { /* use GenerateTypeSafeMetadata() copy instead */ }

static bool IsMetadataWritable(AvaloniaPropertyMetadata m) => !m.IsReadOnly;

When it happens

Trigger: Calling metadata.Merge(baseMetadata, property) on an AvaloniaPropertyMetadata whose Freeze() method has already run (setting IsReadOnly=true). This occurs during OverrideMetadata calls or manual metadata merging after the property registration sequence has frozen the metadata.

Common situations: Overriding styled/direct property metadata at runtime after the property has been sealed; attempting to call OverrideMetadata more than once; sharing a single metadata instance across multiple AvaloniaProperty registrations and then trying to merge into it again.

Related errors

• Metadata is already set for {Name} on {type}.
• Property '{property.Name} not registered on '{o.GetType()}
• Attached properties not supported.
• Cannot register a non-attached property as attached.
• Duration value cannot be negative.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/3e59ccd824156dd9. Report an issue: GitHub.