unoplatform/uno · error · XamlGenerationException

Unable to find class name for toplevel control

Error message

Unable to find class name for toplevel control

What it means

Thrown by GetClassName() when the top-level (root) XAML element has no x:Class attribute. GetClassName is called at line 374 for every non-ResourceDictionary root element; it splits the x:Class value into namespace and class name to generate the partial class. Without x:Class the generator cannot synthesize the code-behind partial class, so generation aborts. This is an XamlGenerationException rooted at the top-level control node.

Source

Thrown at src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs:2345

		private XamlMemberDefinition GetMember(XamlObjectDefinition xamlObjectDefinition, string memberName)
		{
			var member = FindMember(xamlObjectDefinition, memberName);

			if (member == null)
			{
				throw new XamlGenerationException($"'{memberName}' is not defined on '{xamlObjectDefinition.Type.Name}'", xamlObjectDefinition);
			}

			return member;
		}

		private XClassName GetClassName(XamlObjectDefinition control)
		{
			var classMember = FindClassName(control);

			if (classMember == null)
			{
				throw new XamlGenerationException("Unable to find class name for toplevel control", control);
			}

			return classMember;
		}

		private XClassName? FindClassName(XamlObjectDefinition control)
		{
			var classMember = control.Members.FirstOrDefault(m => m.Member.Name == "Class");

			if (classMember?.Value != null)
			{
				var fullName = classMember.Value.ToString() ?? "";

				var index = fullName.LastIndexOf('.');

				return new(fullName.Substring(0, index), fullName.Substring(index + 1), _metadataHelper.FindTypeByFullName(fullName) as INamedTypeSymbol);
			}
			else

View on GitHub (pinned to 0418340488)

Solutions

  1. Add x:Class="YourNamespace.YourClass" to the root element matching the code-behind partial class.
  2. Verify the xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" declaration is present on the root and unmodified.
  3. If the file is meant to be a standalone ResourceDictionary root, confirm it is being treated as one (ResourceDictionary as root) — otherwise it needs x:Class.
  4. Ensure the namespace in x:Class matches the code-behind file's namespace exactly.

Example fix

<!-- before -->
<Page
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
  <Grid/>
</Page>

<!-- after -->
<Page
    x:Class="MyApp.Views.MyPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Grid/>
</Page>
Defensive patterns

Strategy: validation

Validate before calling

# Verify every Page/UserControl/Window XAML root has x:Class
Get-ChildItem -Recurse -Filter *.xaml | Where-Object {
  (Select-String -Path $_.FullName -Pattern '<(Page|UserControl|Window|ContentDialog)' -List) -and
  -not (Select-String -Path $_.FullName -Pattern 'x:Class=' -List)
} | ForEach-Object { Write-Warning "$($_.FullName): root element missing x:Class" }

Prevention

When it happens

Trigger: A Page, UserControl, or Window XAML file whose root element omits x:Class="MyApp.MyPage"; the x:Class attribute is present but on a non-root element; the root uses a custom xmlns that maps x: to a different namespace so the generator does not recognize the Class member.

Common situations: Creating a XAML file from a template that strips x:Class; renaming the code-behind class without updating XAML; mixing up a loose ResourceDictionary XAML (which intentionally has no x:Class) with a Page XAML; a broken xmlns:x declaration.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/234fda5bd831e247. Report an issue: GitHub.