unoplatform/uno · error · NotSupportedException

Attempt to set value from read-only property {0}

Error message

Attempt to set value from read-only property {0}

What it means

Thrown by XamlMemberInvoker.SetValue when the member has no UnderlyingSetter. This happens for read-only properties (no setter) and for events that cannot be set via reflection. The invoker refuses to write a value it has no method to write with.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlMemberInvoker.cs:83

		{
			ThrowIfUnknown ();
			if (instance == null)
				throw new ArgumentNullException ("instance");
			if (member is XamlDirective)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "not supported operation on directive member {0}", member));
			if (UnderlyingGetter == null)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Attempt to get value from write-only property or event {0}", member));
			return UnderlyingGetter.Invoke (instance, Array.Empty<object>());
		}
		public virtual void SetValue (object instance, object value)
		{
			ThrowIfUnknown ();
			if (instance == null)
				throw new ArgumentNullException ("instance");
			if (member is XamlDirective)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "not supported operation on directive member {0}", member));
			if (UnderlyingSetter == null)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Attempt to set value from read-only property {0}", member));
			if (member.IsAttachable)
				UnderlyingSetter.Invoke (null, new object [] {instance, value});
			else
				UnderlyingSetter.Invoke (instance, new object [] {value});
		}

		public virtual ShouldSerializeResult ShouldSerializeValue (object instance)
		{
			throw new NotImplementedException ();
		}
	}
}

View on GitHub (pinned to 0418340488)

Solutions

1. Do not attempt to set read-only members from XAML; mark them with DesignerSerializationVisibility.Hidden or [Bindable(false)] if appropriate.
2. Check `member.Invoker.UnderlyingSetter != null` (or member.IsWriteVisible) before calling SetValue.
3. Add a public setter to the property if it should be XAML-settable.
4. If trimming removed the setter, ensure the type/property is preserved via DynamicDependency or trimming annotations.

Example fix

// before
member.Invoker.SetValue(instance, value);

// after
if (member.Invoker.UnderlyingSetter == null) {
    // read-only or event member; skip or log
    return;
}
member.Invoker.SetValue(instance, value);

// before SetValue
if (member.Invoker.UnderlyingSetter == null) {
    // read-only or event member: skip, log, or report
    return;
}

When it happens

Trigger: Calling SetValue on a XamlMember backed by a get-only property, a computed property, or an event member. Also when a property's setter is non-public and the XamlMember was built without reflecting the setter.

Common situations: XAML that tries to assign a read-only CLR property; binding to a property with only a getter; version changes where a previously settable property became read-only; reflecting internal/private setters under trimming where the setter was removed.

Related errors

• Current operation is valid only when the underlying type on
• The collection type '{0}' does not have 'Add' method
• type
• instance
• item

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/4a8d95420cfe08d7. Report an issue: GitHub.