unoplatform/uno · error · XamlGenerationException

The type '{topLevelControl.Type.Name}' does not support mult

Error message

The type '{topLevelControl.Type.Name}' does not support multiple children

What it means

Thrown in BuildProperties when a Border element has more than one object in its implicit content. Border exposes a single Child property (not a Children collection), so only one child element is permitted. The check at line 2489 fires after the generator confirms the type is a Border and finds multiple objects inside it.

Source

Thrown at src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs:2489

							}
						}
					}
					else if (IsNativePage(topLevelControlSymbol))
					{
						if (implicitContentChild.Objects.Count > 0)
						{
							writer.AppendLineInvariantIndented("{0}Content = ", setterPrefix);

							BuildChild(writer, implicitContentChild, implicitContentChild.Objects.First());
						}
					}
					else if (IsBorder(topLevelControlSymbol))
					{
						if (implicitContentChild.Objects.Count > 0)
						{
							if (implicitContentChild.Objects.Count > 1)
							{
								throw new XamlGenerationException($"The type '{topLevelControl.Type.Name}' does not support multiple children", implicitContentChild.Objects[1]);
							}

							writer.AppendLineInvariantIndented("{0}Child = ", setterPrefix);

							var implicitContent = implicitContentChild.Objects.First();
							using (TryAdaptNative(writer, implicitContent, Generation.UIElementSymbol.Value))
							{
								BuildChild(writer, implicitContentChild, implicitContent);
							}
						}
					}
					else if (IsType(topLevelControlSymbol, Generation.SolidColorBrushSymbol.Value))
					{
						if (implicitContentChild.Value is string content && !content.IsNullOrWhiteSpace())
						{
							writer.AppendLineIndented($"{setterPrefix}Color = {BuildColor(content)}");
						}
					}

View on GitHub (pinned to 0418340488)

Solutions

Wrap the multiple children in a single container such as <Grid> or <StackPanel> inside the Border.
Remove all but one child element if only one is intended.
If you need a border around multiple items, use Border > Grid > (children).

Example fix

<!-- before -->
<Border BorderBrush="Red" BorderThickness="1">
  <TextBlock Text="A"/>
  <TextBlock Text="B"/>
</Border>

<!-- after -->
<Border BorderBrush="Red" BorderThickness="1">
  <StackPanel>
    <TextBlock Text="A"/>
    <TextBlock Text="B"/>
  </StackPanel>
</Border>

Defensive patterns

Strategy: validation

Validate before calling

# Heuristic: flag <Border> elements with more than one direct child element
# (parse with [xml] and count element children of Border)
Get-ChildItem -Recurse -Filter *.xaml | ForEach-Object {
  $x = [xml](Get-Content $_.FullName)
  $x.SelectNodes('//*[local-name()="Border"]') | ForEach-Object {
    $kids = $_.ChildNodes | Where-Object { $_.NodeType -eq 'Element' }
    if ($kids.Count -gt 1) { Write-Warning "$($_.Name) Border has $($kids.Count) children" }
  }
}

Prevention

Remember Border has a single Child, not Children — wrap multiple items in a Grid/StackPanel.
When converting a Grid to a Border, immediately wrap existing children in a panel.

When it happens

Trigger: Writing two or more child elements directly inside a <Border> element (e.g. <Border><TextBlock/><Button/></Border>); placing content plus another element without an explicit property wrapper.

Common situations: Expecting Border to behave like a Panel (Grid/StackPanel) that accepts many children; converting a Grid to a Border and forgetting to wrap the children; copy-paste adding a second element inside an existing Border.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/1a55314d24f93bff. Report an issue: GitHub.