What causes "NotImplementedException" in dotnet/wpf?

Avoid calling GetNamespacePrefixes() inside deferred template content; use GetNamespace(prefix) with a known prefix instead.

dotnet/wpf · error · NotImplementedException

NotImplementedException

Error message

NotImplementedException

What it means

TemplateContent's internal service provider implements IXamlNamespaceResolver but only supports GetNamespace(prefix); GetNamespacePrefixes() is deliberately unimplemented and always throws NotImplementedException. Any consumer asking for the full prefix enumeration gets this exception by design.

Solutions

Avoid calling GetNamespacePrefixes() inside deferred template content; use GetNamespace(prefix) with a known prefix instead.
Obtain namespace prefixes from the outer/primary XAML parser context rather than the template's service provider.
Guard the call in try/catch on NotImplementedException when the service provider is unknown.

Example fix

// before
var prefixes = ((IXamlNamespaceResolver)services.GetService(typeof(IXamlNamespaceResolver))).GetNamespacePrefixes();
// after
var nsResolver = (IXamlNamespaceResolver)services.GetService(typeof(IXamlNamespaceResolver));
var ns = nsResolver.GetNamespace("x"); // only GetNamespace is supported

Defensive patterns

Strategy: try-catch

Validate before calling

var svc = services.GetService(typeof(IXamlNamespaceResolver)) as IXamlNamespaceResolver;
bool hasService = svc != null; // then prefer GetNamespace(prefix) only

Type guard

static bool SupportsPrefixEnumeration(IXamlNamespaceResolver r) { try { r.GetNamespacePrefixes(); return true; } catch (NotImplementedException) { return false; } }

Try / catch

try { prefixes = nsResolver.GetNamespacePrefixes(); }
catch (NotImplementedException) { prefixes = Enumerable.Empty<NamespaceDeclaration>(); }

Prevention

Inside deferred template content use GetNamespace(prefix), not prefix enumeration
Get prefix lists from the root XAML parser context
Defensively treat optional IXaml services as partially implemented

When it happens

Trigger: Calling IXamlNamespaceResolver.GetNamespacePrefixes() on the service object obtained from TemplateContent's IServiceProvider (e.g. via GetService(typeof(IXamlNamespaceResolver))).

Common situations: Custom markup extensions, value serializers, or schema context code that enumerates all namespace prefixes while template content is being deferred-parsed.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c861beb1684d05fc. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/TemplateContent.cs:1481

            {
                FrugalObjectList<NamespaceDeclaration> namespaces = Frames.InScopeNamespaces;
                if (namespaces != null)
                {
                    for (int idx = 0; idx < namespaces.Count; idx++)
                    {
                        if (namespaces[idx].Prefix == prefix)
                        {
                            return namespaces[idx].Namespace;
                        }
                    }
                }

                return ((IXamlNamespaceResolver)_services.GetService(typeof(IXamlNamespaceResolver))).GetNamespace(prefix);
            }

            IEnumerable<NamespaceDeclaration> IXamlNamespaceResolver.GetNamespacePrefixes()
            {
                throw new NotImplementedException();
            }

            #endregion

            #region ITypeDescriptorContext Members

            IContainer ITypeDescriptorContext.Container
            {
                get { return null; }
            }

            object ITypeDescriptorContext.Instance
            {
                get { return null; }
            }

            void ITypeDescriptorContext.OnComponentChanged()
            {

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