What causes "throw new NotImplementedException();" in dotnet/wpf?

Only call Clone on frame types known to override it (e.g. ObjectWriterFrame)

dotnet/wpf · warning · NotImplementedException

throw new NotImplementedException();

Error message

throw new NotImplementedException();

What it means

XamlFrame.Clone is a virtual placeholder that intentionally throws NotImplementedException; only subclasses that need cloning (e.g. ObjectWriterFrame) override it. Calling Clone on a base XamlFrame (or a subclass that never overrides) is unsupported by design.

Solutions

Only call Clone on frame types known to override it (e.g. ObjectWriterFrame)
Check the concrete type before calling Clone, or add an override in your frame subclass
Use frame Reset() on the existing stack instead of cloning when reuse is the goal
Wrap in try-catch for NotImplementedException when frames are processed generically

Example fix

// before
var copy = frame.Clone();
// after
var copy = frame is ObjectWriterFrame owf ? owf.Clone() : null;

Defensive patterns

Strategy: fallback

Validate before calling

static bool SupportsClone(MS.Internal.Xaml.Context.XamlFrame f) =>
    f.GetType().GetMethod("Clone").DeclaringType != f.GetType().GetTypeInfo().BaseType && f.GetType().GetMethod("Clone").IsOverridden();
// simpler: f is MS.Internal.Xaml.Context.ObjectWriterFrame

Type guard

static bool CanClone(XamlFrame f) => f is ObjectWriterFrame;

Try / catch

try { copy = frame.Clone(); }
catch (NotImplementedException) { /* fall back to Reset() reuse or skip cloning */ }

Prevention

Only clone frame types that document a Clone override
Use Reset() for frame reuse instead of cloning
When adding XamlFrame subclasses, override Clone if instances will be copied

When it happens

Trigger: Invoking Clone() on a XamlFrame instance whose concrete type does not override Clone — typically the abstract base or a lightweight frame type used in the XAML parser stack.

Common situations: Custom XAML parser/object-writer extensions calling Clone generically on any frame in the stack; reflection-based code copying frames without checking the concrete type.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/bc5f458bad2b993a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/Xaml/Context/XamlFrame.cs:29

        private XamlFrame _previous;

        protected XamlFrame()
        {
            _depth = -1;
        }

        // Copy constructor
        protected XamlFrame(XamlFrame source)
        {
            _depth = source._depth;
        }

        public virtual XamlFrame Clone()
        {
            // Clone should only be overridden for the classes that really need it
            // ObjectWriterFrame overrides this so we can reuse the context for
            // Templates.
            throw new NotImplementedException();
        }

        // Reset the contents of the Frame so it can be reused in a stack without reallocating.
        // Depth and previous do not change when we reuse the Frame.
        public abstract void Reset();

        public int Depth
        {
            get
            {
                Debug.Assert(_depth != -1, "Context Frame is uninitialized");
                return _depth;
            }
        }

        public XamlFrame Previous
        {
            get { return _previous; }

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