dotnet/wpf · error · XamlSchemaException

' '.' ' is a property without a getter and is not a valid…

Error message

'{0}'.'{1}' is a property without a getter and is not a valid XAML property.

What it means

System.Xaml throws this XamlSchemaException when an attachable member is registered that has neither a getter nor a setter method. XAML properties must be readable to be valid; a member with no accessors cannot participate in XAML loading or type reflection, so the schema layer rejects it at construction time.

Solutions

Define a public static Get<Name>(target) method on the declaring type (or pass a valid MethodInfo as getter).
Ensure the Set<Name>(target, value) method exists and is resolvable so the constructor receives a non-null setter.
Fix the type resolution/naming convention in the code that locates the accessor methods before constructing ClrAttachedProperty.

Example fix

// before: no accessors found
class MyExt { /* no GetGlow / SetGlow */ }
// after
class MyExt {
  public static double GetGlow(DependencyObject o) => ...;
  public static void SetGlow(DependencyObject o, double v) { ... }
}

Defensive patterns

Strategy: validation

Validate before calling

// before registering an attachable property
var getter = type.GetMethod("Get" + name, BindingFlags.Public|BindingFlags.Static, null, new[]{ targetType }, null);
var setter = type.GetMethod("Set" + name, BindingFlags.Public|BindingFlags.Static);
if (getter == null && setter == null)
    throw new InvalidOperationException($"{type.Name}.{name} needs a Get{Name} or Set{Name} static method");

Type guard

bool HasAccessor(EventInfo e) => e.GetAddMethod(true) != null;
// for attachable props: getter != null || setter != null

Try / catch

try { schema.RegisterAttachableProperty(name, type); }
catch (XamlSchemaException ex) { log($"Invalid attachable property: {ex.Message}"); }

Prevention

Always pair attachable properties with Get/Set static methods following the WPF naming convention.
Unit-test XAML schema registration for every attachable member.
Run reflection checks over the assembly before loading XAML.

When it happens

Trigger: Calling the internal/public ClrAttachedProperty constructor (via reflection-based schema provider) with getter == null and setter == null for an attachable property of declaringType.

Common situations: Custom XAML schema/context implementations registering attachable properties discovered by convention, where naming-based reflection failed to bind either an Get<Name> or Set<Name> static method; refactoring renamed one accessor so neither resolved.

Understand the failure class

Background: "not installed", "pip install", "required for": how missing-dependency errors surface across open-source libraries — this error's family across 34 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/858b614dde20b74a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/ClrAttachableProperty.cs:37

namespace MS.Internal.Xaml.Schema
#else
namespace System.Xaml.Schema
#endif
{
    [DebuggerDisplay("{Name}")]
    class ClrAttachedProperty : ClrProperty
    {
        public readonly MethodInfo ClrBindingGetterMethodInfo;
        public readonly MethodInfo ClrBindingSetterMethodInfo;
        private Type _systemTypeOfProperty = null;
        internal ClrAttachedProperty(string name, MethodInfo getter, MethodInfo setter, XamlType declaringType)
            : base(name, declaringType)
        {
            Debug.Assert(getter != null || setter != null);

            if (getter == null && setter == null)
            {
                throw new XamlSchemaException(SR.Format(SR.SetOnlyProperty, declaringType.Name, name));
            }

            _isPublic = (getter != null) ? getter.IsPublic : setter.IsPublic;
            _isReadOnly = (setter == null);
            _isStatic = false;
            _isAttachable = true;
            _isEvent = false;

            ClrBindingGetterMethodInfo = getter;
            ClrBindingSetterMethodInfo = setter;
        }

        protected override Type LookupSystemTypeOfProperty()
        {
            if (_systemTypeOfProperty == null)
            {
                _systemTypeOfProperty = PrivateLookupSystemTypeOfProperty;
            }

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