AvaloniaUI/Avalonia · error · ArgumentException

The property {property.Name} is readonly.

Error message

The property {property.Name} is readonly.

What it means

Thrown by AvaloniaObject.ThrowIfReadOnly when attempting to write a property whose IsReadOnly flag is set. ReadOnly properties (typically registered with RegisterReadOnly and exposing only a public getter) reject local writes and bindings.

Source

Thrown at src/Avalonia.Base/AvaloniaObject.cs:888

            }
            else
            {
                builder.Append(ToString());
            }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void ValidatePriority(BindingPriority priority)
        {
            if (priority < BindingPriority.Animation || priority >= BindingPriority.Inherited)
                ThrowInvalidPriority(priority);
        }

        private static void ThrowIfReadOnly(AvaloniaProperty property)
        {
            if (property.IsReadOnly)
            {
                throw new ArgumentException($"The property {property.Name} is readonly.");
            }
        }

        private static void ThrowInvalidPriority(BindingPriority priority)
        {
            throw new ArgumentException($"Invalid priority ${priority}", nameof(priority));
        }
    }
}

View on GitHub (pinned to 11c5427268)

Solutions

1. Do not bind or set read-only properties; use the control's API that internally updates them.
2. If you own the property and need external writes, register it as read-write or expose a protected setter key.
3. In XAML/styles, remove setters that target read-only properties.

Example fix

// before
control.SetValue(Visual.BoundsProperty, new Rect(...)); // Bounds is read-only

// after
// drive the bounds through layout, not by setting the property
control.InvalidateMeasure();

if (property.IsReadOnly)
    throw new InvalidOperationException($"{property.Name} is read-only; use the control API instead");

static bool IsWritable(AvaloniaProperty p) => !p.IsReadOnly;

try { obj.SetValue(property, value); }
catch (ArgumentException ex) when (ex.Message.Contains("readonly"))
{ /* cannot write; adjust caller */ }

When it happens

Trigger: Calling SetValue/Bind on a ReadOnlyProperty; XAML or styling trying to assign a value to a read-only property; a style setter targeting a read-only property.

Common situations: Confusing a read-only property (computed/managed by the control) with a settable one; targeting properties like Visual.Bounds or control computed-state in styles.

Related errors

• Unsupported AvaloniaProperty type.
• Invalid priority ${priority}
• 'name' may not contain periods.
• The property {Name} is readonly.
• Transition elements have different parents.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/b75b6143171b3aa8. Report an issue: GitHub.