unoplatform/uno · error · InvalidOperationException

Unable to find type {fullyQualifiedName}

Error message

Unable to find type {fullyQualifiedName}

What it means

Thrown (lazily) by GetMandatorySymbolAsLazy when Compilation.GetTypeByMetadataName returns null for a type that the XAML generator considers mandatory — including Border, SolidColorBrush, RowDefinition, ColumnDefinition, and System.Threading.Tasks.Task. The Lazy is only evaluated when the symbol is first accessed.

Source

Thrown at src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlCodeGeneration.cs:375

			WindowSymbol = GetOptionalSymbolAsLazy(XamlConstants.Types.Window);
			ApplicationSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.Application);
			ResourceDictionarySymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.ResourceDictionary);
			TextBlockSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.TextBlock);
			RunSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.Run);
			SpanSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.Span);
			BorderSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.Border);
			SolidColorBrushSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.SolidColorBrush);
			RowDefinitionSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.RowDefinition);
			ColumnDefinitionSymbol = GetMandatorySymbolAsLazy(XamlConstants.Types.ColumnDefinition);
			TaskSymbol = GetMandatorySymbolAsLazy("System.Threading.Tasks");

			TypeProviders = ImmutableArray.Create<ITypeProvider>(
				new MvvmTypeProvider(this)
				);
		}

		private Lazy<INamedTypeSymbol> GetMandatorySymbolAsLazy(string fullyQualifiedName)
			=> new(() => _generatorContext.Compilation.GetTypeByMetadataName(fullyQualifiedName) ?? throw new InvalidOperationException($"Unable to find type {fullyQualifiedName}"));

		internal Lazy<INamedTypeSymbol?> GetOptionalSymbolAsLazy(string fullyQualifiedName)
			=> new(() => _generatorContext.Compilation.GetTypeByMetadataName(fullyQualifiedName));

		private Lazy<INamedTypeSymbol> GetSpecialTypeSymbolAsLazy(SpecialType specialType)
			=> new(() => _generatorContext.Compilation.GetSpecialType(specialType));

		private static bool IsWinUIItem(Uno.Roslyn.MSBuildItem item)
			=> item.GetMetadataValue("XamlRuntime") is { } xamlRuntime
				? xamlRuntime == "WinUI" || string.IsNullOrWhiteSpace(xamlRuntime)
				: true;

		private IEnumerable<Uno.Roslyn.MSBuildItem> GetWinUIItems(string name)
			=> _generatorContext.GetMSBuildItemsWithAdditionalFiles(name).Where(IsWinUIItem);

		/// <summary>
		/// Get the file location as seen in the IDE, used for ResourceDictionary.Source resolution.
		/// </summary>

View on GitHub (pinned to 0418340488)

Solutions

1. Verify that the project references the correct Uno.UI (and related) NuGet packages with compatible versions.
2. Run dotnet restore to ensure all packages are properly restored.
3. Check for version mismatches between Uno.UI, Uno.WinUI, and the target framework packages.
4. If the error names a specific type, ensure the assembly defining that type is referenced.

Example fix

<!-- Ensure correct package references in .csproj -->
<ItemGroup>
  <PackageReference Include="Uno.UI" Version="5.x.x" />
  <PackageReference Include="Uno.WinUI" Version="5.x.x" />
</ItemGroup>

<!-- Verify mandatory type-providing packages are referenced -->
<ItemGroup>
  <PackageReference Include="Uno.WinUI" Version="$(UnoVersion)" />
  <PackageReference Include="Uno.UI" Version="$(UnoVersion)" />
</ItemGroup>
<!-- After editing, run: dotnet restore && dotnet build -->

When it happens

Trigger: The Roslyn compilation used by the source generator does not include a reference to the assembly that defines one of the mandatory types. For example, if Uno.UI or the WinUI reference assembly providing Border/SolidColorBrush is not referenced.

Common situations: A project references Uno.UI NuGet packages incorrectly or uses incompatible versions; the compilation is missing the WinUI/Uno type-forwarding assembly; package downgrade or restore failure left required assemblies unresolvable.

Related errors

• The type '{fullyQualifiedMetadataName}' is not found.
• Unable to resolve {symbol} (Reason: {errorTypeSymbol.Candida
• No valid setter found for attached property {name}
• Unable to find type {fullName}
• The type {0} could not be found

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/da64cb132485fc43. Report an issue: GitHub.