dotnet/maui · error · BuildException

XC0065

XC0065

Error message

Cannot set the content of "{0}" as it doesn't have a [ContentProperty] attribute.

What it means

Thrown when a child element is placed directly in the content of a parent element (collection-item / implicit-content syntax) but the parent's type has no [ContentProperty] attribute and no applicable single-parameter Add method. Without the attribute the compiler cannot determine which property the inline content maps to.

Source

Thrown at src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs:161

					Context.IL.Append(SetPropertyValue(Context.Variables[(ElementNode)parentNode], name, node, Context, node));
				}
				// Collection element, implicit content, or implicit collection element.
				else if (parentVar.VariableType.ImplementsInterface(Context.Cache, Module.ImportReference(Context.Cache, ("mscorlib", "System.Collections", "IEnumerable")))
						 && parentVar.VariableType.GetMethods(Context.Cache, md => md.Name == "Add" && md.Parameters.Count == 1, Module).Any())
				{
					var elementType = parentVar.VariableType;
					var adderTuple = elementType.GetMethods(Context.Cache, md => md.Name == "Add" && md.Parameters.Count == 1, Module).First();
					var adderRef = Module.ImportReference(adderTuple.Item1);
					adderRef = Module.ImportReference(adderRef.ResolveGenericParameters(adderTuple.Item2, Module));

					Context.IL.Emit(Ldloc, parentVar);
					Context.IL.Append(vardef.LoadAs(Context.Cache, adderRef.Parameters[0].ParameterType.ResolveGenericParameters(adderRef), Module));
					Context.IL.Emit(Callvirt, adderRef);
					if (adderRef.ReturnType.FullName != "System.Void")
						Context.IL.Emit(Pop);
				}
				else
					throw new BuildException(ContentPropertyAttributeMissing, node, null, ((ElementNode)parentNode).XmlType.Name);
			}
			else if (IsCollectionItem(node, parentNode) && parentNode is ListNode node2)
			{
				//				IL_000d:  ldloc.2 
				//				IL_000e:  callvirt instance class [mscorlib]System.Collections.Generic.IList`1<!0> class [Microsoft.Maui.Controls]Microsoft.Maui.Controls.Layout`1<class [Microsoft.Maui.Controls]Microsoft.Maui.Controls.View>::get_Children()
				//				IL_0013:  ldloc.0 
				//				IL_0014:  callvirt instance void class [mscorlib]System.Collections.Generic.ICollection`1<class [Microsoft.Maui.Controls]Microsoft.Maui.Controls.View>::Add(!0)

				var parentList = node2;
				var parent = Context.Variables[(ElementNode)parentList.Parent];

				if (skips.Contains(parentList.XmlName))
					return;
				if (parentNode is ElementNode node3 && node3.SkipProperties.Contains(propertyName))
					return;
				var elementType = parent.VariableType;
				var localname = parentList.XmlName.LocalName;

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Add [ContentProperty(nameof(ChildrenProperty))] to the parent type so inline content maps to a property.
2. Wrap the child in an explicit property element, e.g. `<MyControl.Content><Label/></MyControl.Content>`.
3. Use a control/collection type that already declares a content property or implements Add(T).

Example fix

<!-- before -->
<MyPanel>
  <Label />
</MyPanel>

<!-- after: add attribute in C# -->
[ContentProperty(nameof(Children))]
public class MyPanel : Layout<View> { /* ... */ }

<!-- or wrap explicitly -->
<MyPanel>
  <MyPanel.Children>
    <Label />
  </MyPanel.Children>
</MyPanel>

// Verify a type is usable as XAML content before putting children inline
static bool SupportsXamlContent(Type t)
    => t.GetCustomAttributes(true)
        .Any(a => a.GetType().FullName == "System.Windows.Markup.ContentPropertyAttribute");

When it happens

Trigger: A custom control or element used as a container with inline children, where the type lacks [ContentProperty(...)] and is not itself an IEnumerable-with-Add, e.g. `<MyControl><Label/></MyControl>` on a type with no ContentProperty attribute.

Common situations: Authoring a custom ContentView/Element and forgetting the [ContentProperty] attribute; using a type from another assembly that was not attributed for XAML content.

Related errors

• XC0020
• XC0040
• XC0000
• XC0001
• Expected VisualStateGroup but found {parent.Parent}

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/3c1fd2f964c509af. Report an issue: GitHub.