dotnet/wpf · error · NotImplementedException

NotImplementedException(SR.MissingCaseXamlNodes)

Error message

NotImplementedException(SR.MissingCaseXamlNodes)

What it means

XamlWriter.WriteNode's switch over XamlNodeType has no handler for the node type it received, so it hits the default case and throws NotImplementedException. This is an internal exhaustiveness guard: the writer supports only the concrete XAML node kinds (Start/End Object, Start/End Member, Value, Namespace, None), and anything else is unsupported.

Solutions

1. Ensure the source IXamlReader only emits standard XamlNodeType values; fix or replace the custom reader.
2. Filter or transform unexpected nodes (e.g. via XamlNodeQueue/XamlNodeEnumerator) before calling WriteNode.
3. Update System.Xaml/pipeline components so reader and writer node-type enums match.
4. Add an explicit case/handling if you own a node type extension point.

Example fix

// before
while (reader.Read()) writer.WriteNode(reader); // NotImplementedException on unknown node
// after
while (reader.Read())
{
    if (reader.NodeType == XamlNodeType.None) continue; // skip unsupported nodes
    writer.WriteNode(reader);
}

if (node is null || !Enum.IsDefined(typeof(XamlNodeType), node.NodeType))
    throw new InvalidOperationException("Unsupported XAML node type in stream");

bool IsWritableNode(IXamlReader r) => r.NodeType is XamlNodeType.StartObject or XamlNodeType.EndObject
    or XamlNodeType.StartMember or XamlNodeType.EndMember or XamlNodeType.Value or XamlNodeType.Namespace or XamlNodeType.None;

try { writer.WriteNode(reader); }
catch (NotImplementedException ex) { /* log node type; filter/skip node or fix custom reader */ }

When it happens

Trigger: Feeding a XamlReader-produced node stream containing a node type WriteNode doesn't handle into XamlServices/xamlWriter.WriteNode; passing XamlNodeType.None-like or corrupted nodes from a custom IXamlReader implementation.

Common situations: Custom XAML readers/writers that emit non-standard node types; mixing nodes from different System.Xaml versions; piping a reader's GetReader results through a transform into XamlXmlWriter with an unexpected node in the stream.

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/350c63d4c23d33ac. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlWriter.cs:59

                break;

            case XamlNodeType.StartMember:
                WriteStartMember(reader.Member);
                break;

            case XamlNodeType.EndMember:
                WriteEndMember();
                break;

            case XamlNodeType.Value:
                WriteValue(reader.Value);
                break;

            case XamlNodeType.None:
                break;

            default:
                throw new NotImplementedException(SR.MissingCaseXamlNodes);
            }
        }

        #region IDisposable

        // See Framework Design Guidelines, pp. 254-256.

        protected bool IsDisposed { get; private set; }

        void IDisposable.Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected virtual void Dispose(bool disposing)
        {
            IsDisposed = true; // must call the base class to get IsDisposed == true;

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