unoplatform/uno · error · ArgumentException

Property getter for {0} must have exactly one argument and m

Error message

Property getter for {0} must have exactly one argument and must have non-void return type.

What it means

Thrown by XamlMember.VerifyGetter when an attachable property getter MethodInfo does not match the expected signature: exactly one parameter and a non-void return type. Attachable property getters follow the static GetX(object target) pattern; a method with a different signature cannot serve as one and is rejected.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlMember.cs:468

			return underlying_setter;
		}

		protected virtual XamlValueConverter<ValueSerializer> LookupValueSerializer ()
		{
			if (is_predefined_directive) // FIXME: this is likely a hack.
				return null;
			if (Type == null)
				return null;

			return XamlType.LookupValueSerializer (Type, LookupCustomAttributeProvider ()) ?? Type.ValueSerializer;
		}

		void VerifyGetter (MethodInfo method)
		{
			if (method == null)
				return;
			if (method.GetParameters ().Length != 1 || method.ReturnType == typeof (void))
				throw new ArgumentException (String.Format (CultureInfo.InvariantCulture, "Property getter for {0} must have exactly one argument and must have non-void return type.", Name));
		}

		void VerifyAdderSetter (MethodInfo method)
		{
			if (method == null)
				return;
			if (method.GetParameters ().Length != 2)
				throw new ArgumentException (String.Format (CultureInfo.InvariantCulture, "Property getter or event adder for {0} must have exactly one argument and must have non-void return type.", Name));
		}
	}
}

View on GitHub (pinned to 0418340488)

Solutions

1. Pass the correct static attachable getter: a method with signature static T GetPropName(object target) — exactly one parameter, non-void return.
2. Filter reflection results: type.GetMethods().First(m => m.Name == "Get"+propName && m.GetParameters().Length == 1 && m.ReturnType != typeof(void)).
3. Do not pass instance property accessors to the attachable-property XamlMember constructor.

Example fix

// before
var getter = type.GetMethod("GetName"); // picks instance getter with 0 params
var member = new XamlMember("Name", getter, setter, sctx);

// after
var getter = type.GetMethods()
    .First(m => m.Name == "GetName" && m.IsStatic && m.GetParameters().Length == 1);
var member = new XamlMember("Name", getter, setter, sctx);

var getter = type.GetMethods()
    .FirstOrDefault(m => m.Name == "Get"+propName && m.IsStatic
        && m.GetParameters().Length == 1 && m.ReturnType != typeof(void));
if (getter == null) return null;
var member = new XamlMember(propName, getter, setter, sctx);

static bool IsValidAttachableGetter(MethodInfo m)
    => m.IsStatic && m.GetParameters().Length == 1 && m.ReturnType != typeof(void);

try { VerifyGetterIsOk(getter); new XamlMember(...); }
catch (ArgumentException) { /* find correct overload */ }

When it happens

Trigger: Passing a MethodInfo to the XamlMember attachable constructor where the getter takes zero or two+ parameters, or returns void. E.g. passing an instance property getter (0 params) instead of the static attachable accessor.

Common situations: Reflection code that picks the wrong overload of a method named Get<PropName> (e.g. a no-arg instance getter vs. the static one-arg attachable accessor). Confusing attachable property accessors with regular instance property accessors.

Related errors

• Property getter or event adder for {0} must have exactly one
• Either of converterType, targetType or name must be non-null
• Either property getter or setter must be non-null.
• Attempt to set value from read-only property {0}
• Current operation is valid only when the underlying type on

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/a347d300f4c2eeca. Report an issue: GitHub.