unoplatform/uno · error · XamlGenerationException

The property '{member.Member.Name}' of type '{propertyType}'

Error message

The property '{member.Member.Name}' of type '{propertyType}' does not support adding multiple objects

What it means

Companion to 310 in the same branch: thrown when a property is single-valued (not a collection, no Add support) and the XAML provides more than one object inside it. The property type cannot accept multiple objects, so generation aborts at line 3461. This is the explicit-property-element version of the 'multiple children on a scalar' problem (cf. 305 which is the implicit-content version).

Source

Thrown at src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs:3461

									}

									foreach (var inner in member.Objects)
									{
										writer.AppendIndented($"{localCollectionName}.Add(");

										BuildChild(writer, member, inner);

										writer.AppendLineIndented(");");
									}
								}
								else if (member.Objects.Count == 1)
								{
									var childType = GetType(member.Objects[0].Type).GetFullyQualifiedTypeExcludingGlobal();
									throw new XamlGenerationException($"Cannot assign child of type '{childType}' to property of type '{propertyType.GetFullyQualifiedTypeExcludingGlobal()}'", member.Objects[0]);
								}
								else
								{
									throw new XamlGenerationException($"The property '{member.Member.Name}' of type '{propertyType}' does not support adding multiple objects", member);
								}
							}
							else
							{
								// GenerateWarning(writer, $"Unknown type {objectDefinition.Type} for property {member.Member.DeclaringType}");
							}
						}
						else
						{
							var isMemberInsideResourceDictionary = IsMemberInsideResourceDictionary(objectDefinition);
							var value = member.Value?.ToString();

							if (
								member.Member.Name == "Name"
								&& member.Member.PreferredXamlNamespace == XamlConstants.XamlXmlNamespace
								&& !isMemberInsideResourceDictionary.isInside
							)
							{

View on GitHub (pinned to 0418340488)

Solutions

  1. Reduce to a single object inside the property-element tag.
  2. If multiple values are needed, confirm the property is collection-typed; if not, choose the correct collection property or wrap appropriately.
  3. Move extra objects to a sibling property-element that accepts them.

Example fix

<!-- before -->
<Button>
  <Button.Content>
    <TextBlock Text="A"/>
    <TextBlock Text="B"/>
  </Button.Content>
</Button>

<!-- after -->
<Button>
  <Button.Content>
    <StackPanel>
      <TextBlock Text="A"/>
      <TextBlock Text="B"/>
    </StackPanel>
  </Button.Content>
</Button>
Defensive patterns

Strategy: validation

Validate before calling

# Structural lint: property-element tags with >1 element child where the property is scalar
Get-ChildItem -Recurse -Filter *.xaml | ForEach-Object {
  $x = [xml](Get-Content $_.FullName)
  $x.SelectNodes('//*[contains(local-name(),".")]') | ForEach-Object {
    $kids = $_.ChildNodes | Where-Object { $_.NodeType -eq 'Element' }
    if ($kids.Count -gt 1) { Write-Warning "$($_.LocalName) property-element has $($kids.Count) children" }
  }
}

Prevention

When it happens

Trigger: Writing <MyControl.Title><x/><y/></MyControl.Title> where Title is a string/object (single); nesting two or more elements inside any property-element whose property is not a collection.

Common situations: Assuming a property is a collection when it is scalar; copy-pasting multiple setters into a single Value; porting XAML where a property's multiplicity differs.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/dba45e1fbf92c002. Report an issue: GitHub.