dotnet/wpf · error · NotSupportedException

SR.NotSupportedOnDirective

Error message

SR.NotSupportedOnDirective

What it means

DirectiveMemberInvoker (used for XAML directives like x:Name, x:Key) overrides GetValue to always throw NotSupportedException with SR.NotSupportedOnDirective. Directives are language constructs, not CLR properties, so there is no backing value to read through the invoker.

Solutions

  1. Skip directives when enumerating: filter members where `member is XamlDirective` before calling GetValue.
  2. Handle directive semantics explicitly (e.g. read x:Name from the actual NameScope/name mechanism) instead of through the invoker.
  3. Catch NotSupportedException around GetValue if the member set is not known in advance.

Example fix

// before
foreach (var m in type.GetAllMembers()) Read(m); // throws on x:Name etc.
// after
foreach (var m in type.GetAllMembers())
{
    if (m is XamlDirective) continue;
    Read(m);
}
Defensive patterns

Strategy: type-guard

Validate before calling

if (member is XamlDirective) return; // no value to read

Type guard

static bool IsDirective(XamlMember m) => m is XamlDirective;

Try / catch

try { v = invoker.GetValue(instance); }
catch (NotSupportedException) { /* directive: no CLR value */ }

Prevention

When it happens

Trigger: Calling GetValue on the invoker of a XamlDirective (any member obtained from XamlLanguage.Name, XamlLanguage.Key, XamlLanguage.Class, etc.).

Common situations: Generic member-enumeration code that iterates type.GetAllMembers() (which includes directives) and tries to read each value; serializers dumping object graphs that encounter directive members.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9f479f0a28b324bd. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XamlMemberInvoker.cs:167

        private bool IsUnknown
        {
            get { return _member is null || _member.UnderlyingMember is null; }
        }

        private void ThrowIfUnknown()
        {
            if (IsUnknown)
            {
                throw new NotSupportedException(SR.NotSupportedOnUnknownMember);
            }
        }

        private class DirectiveMemberInvoker : XamlMemberInvoker
        {
            public override object GetValue(object instance)
            {
                throw new NotSupportedException(SR.NotSupportedOnDirective);
            }

            public override void SetValue(object instance, object value)
            {
                throw new NotSupportedException(SR.NotSupportedOnDirective);
            }
        }
    }

    public enum ShouldSerializeResult
    {
        Default,
        True,
        False
    }
}

View on GitHub (pinned to 81131a70a4)