dotnet/wpf · error · NotImplementedException

NotImplementedException

Error message

NotImplementedException

What it means

GetRegularDependencyProperty wraps a DependencyProperty-backed CLR property into a WpfXamlMember, but only when the base lookup returned a PropertyInfo. If the underlying member is not a PropertyInfo, the code has hit an unhandled case and throws a bare NotImplementedException — the internal assumption that DP-backed members are always properties was violated.

Solutions

1. Ensure the type's DependencyProperty-backed members are exposed as real CLR wrapper properties.
2. Check for a name clash: an event or method sharing the name of the DP wrapper is shadowing the property — rename it.
3. If overriding LookupMember in a custom XamlType, return members whose UnderlyingMember is a PropertyInfo for DP-backed names.
4. Catch NotImplementedException in tooling paths and fall back to base XamlType member resolution.

Example fix

// before
public event EventHandler Foo; // alongside public static readonly DependencyProperty FooProperty
// after
public static readonly DependencyProperty FooProperty = DependencyProperty.Register(nameof(Foo), ...);
public object Foo { get => GetValue(FooProperty); set => SetValue(FooProperty, value); } // CLR wrapper is a PropertyInfo

var m = baseType.GetMember(name).FirstOrDefault(x => x is PropertyInfo);
if (m == null) throw new InvalidOperationException($"'{name}' on {baseType} is not a property; DP-backed members must be CLR properties.");

bool IsPropertyBacked(System.Reflection.MemberInfo mi) => mi is PropertyInfo;

try { member = wpfType.GetMember(name); }
catch (NotImplementedException)
{
    // fall back to base XamlType.LookupMember or reflection-based resolution
}

When it happens

Trigger: LookupMember on a WpfXamlType resolves a DependencyProperty-backed name whose base XamlMember.UnderlyingMember is not a PropertyInfo (e.g. an event, a method, or a custom XamlMember returned by an overridden base lookup).

Common situations: Custom XAML schema extensions or third-party XamlType subclasses returning non-property underlying members for names that clash with DependencyProperty fields; dynamic-type scenarios where the member table contains unexpected member kinds.

Related errors

• ArgumentException: path
• ArgumentException: relativeTo
• ArgumentNullException: path
• ArgumentNullException: relativeTo
• NotImplementedException

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/629fb699bcc2d09e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/Baml2006/WpfXamlType.cs:426

                    memberFromBase.Invoker.UnderlyingSetter,
                    SchemaContext, UseV3Rules);
            }
            return null;
        }

        private XamlMember GetRegularDependencyProperty(string name, DependencyProperty property, bool skipReadOnlyCheck)
        {
            XamlMember memberFromBase = base.LookupMember(name, skipReadOnlyCheck);
            if (memberFromBase != null)
            {
                PropertyInfo propertyInfo = memberFromBase.UnderlyingMember as PropertyInfo;
                if (propertyInfo != null)
                {
                    return new WpfXamlMember(property, propertyInfo, SchemaContext, UseV3Rules);
                }
                else
                {
                    throw new NotImplementedException();
                }
            }

            return null;
        }

        // First try looking at the cache
        // Then look to see if we have a known property on the type
        private static XamlMember FindKnownMember(WpfXamlType wpfXamlType, string name, bool isAttachable)
        {
            XamlMember xamlMember = null;

            // Look in the cache first
            if (!isAttachable || wpfXamlType.IsBamlScenario)
            {
                if (wpfXamlType._members != null && wpfXamlType.Members.TryGetValue(name, out xamlMember))
                {
                    return xamlMember;

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