unoplatform/uno · error · InvalidOperationException

No valid setter found for attached property

Error message

No valid setter found for attached property {name}

What it means

To resolve an attached property's value type, the generator walks the owner type's hierarchy looking for a 'Set<Name>' method whose return yields a DependencyProperty type. If no such setter is found up to System.Object, it throws InvalidOperationException. This indicates the attached property name is wrong or the owner does not define it.

Solutions

1. Correct the spelling of the attached property in XAML to match the owner's Set<Name> method.
2. Verify the owner type actually declares that attached property in the referenced WinUI/Uno version.
3. Add the missing xmlns/assembly reference for the owner type.

Example fix

<!-- before -->
<Grid><TextBlock Grid.Columm="1" /></Grid>
<!-- after -->
<Grid><TextBlock Grid.Column="1" /></Grid>

// Verify an attached property exists on its owner before relying on it in XAML
var setter = ownerType.GetMethod("Set" + propertyName, new[] { typeof(DependencyObject), propertyType });
if (setter == null) throw new InvalidOperationException($"No Set{propertyName} on {ownerType}.");

When it happens

Trigger: XAML uses an attached property (e.g. <Type>.<Name>) whose 'Set<Name>' accessor cannot be found on the owner type or any of its base types.

Common situations: A typo in the attached property name in XAML (Grid.Columm vs Grid.Column); referencing an attached property on a type that does not provide it; the owner type's setter was renamed/removed.

Related errors

• The type ' ' is not found.
• Unable to find type
• Unable to resolve (Reason: , Location: , Candidates: )
• Expected attached property path to be member access.
• Expected ')' in attached property syntax.

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/b87e450da657394c. Report an issue: GitHub.

Appendix: source

Thrown at src/SourceGenerators/Uno.UI.SourceGenerators/RoslynMetadataHelper.cs:133

		}

		public INamedTypeSymbol GetAttachedPropertyType(INamedTypeSymbol type, string propertyName)
			=> _getAttachedPropertyType(type, propertyName);

		private static INamedTypeSymbol SourceGetAttachedPropertyType(INamedTypeSymbol? type, string name)
		{
			while (type != null && type.SpecialType != SpecialType.System_Object)
			{
				var setMethod = type?.GetFirstMethodWithName("Set" + name);
				if (setMethod?.FindDependencyPropertyType() is { } propertyType)
				{
					return propertyType;
				}

				type = type?.BaseType;
			}

			throw new InvalidOperationException($"No valid setter found for attached property {name}");
		}

		public ITypeSymbol GetTypeByFullName(string fullName)
		{
			var symbol = FindTypeByFullName(fullName);

			if (symbol == null)
			{
				throw new InvalidOperationException($"Unable to find type {fullName}");
			}

			return symbol;
		}

		public bool IsTypeImplemented(INamedTypeSymbol type) => _isTypeImplemented(type);

		private static bool SourceIsTypeImplemented(INamedTypeSymbol type)
			=> type.GetAttributes().None(a => a.AttributeClass?.GetFullyQualifiedTypeExcludingGlobal() == XamlConstants.Types.NotImplementedAttribute);

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