dotnet/wpf · error · InvalidOperationException

SR.MustNotCallSetter

Error message

SR.MustNotCallSetter

What it means

UnknownProperty.IsDirectiveCore exposes only a getter; its setter unconditionally throws InvalidOperationException with SR.MustNotCallSetter. UnknownProperty is a read-only sentinel member representing unrecognized properties, so overriding these lookup results is not supported — it is an internal invariant against mutating the override hook.

Solutions

1. Do not write to these override properties; provide values via the constructor or by overriding with your own getter-only implementation in a subclass.
2. Use a different XamlProperty-derived type if you need a member that supports these flags.
3. Remove reflection code that sets properties without checking for setter-guard exceptions.

Example fix

// before
member.IsDirectiveCore = true; // throws

// after
class DirectiveProperty : UnknownProperty
{
    protected override bool IsDirectiveCore => true;
}

if (member is UnknownProperty) throw new InvalidOperationException("UnknownProperty override members are read-only");

bool IsMutableMember(XamlMember m) => m is not UnknownProperty;

try { member.IsDirective = flag; }
catch (InvalidOperationException) { /* UnknownProperty and similar sentinels are immutable; use a concrete member type */ }

When it happens

Trigger: Assigning to the protected virtual IsDirectiveCore property (directly or through a base-class setter path) on an UnknownProperty instance, e.g. from a subclass or schema component trying to set the directive flag.

Common situations: Custom XAML schema code deriving from or poking at XamlProperty overrides and attempting to initialize override properties via setters instead of constructor arguments; reflection-based tools writing to properties regardless of setter logic.

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

• Cannot remove signature from read-only file.
• Cannot set read-only property
• Image_OriginalStreamReadOnly
• InvalidOperationException(SR.Format(SR.PrefixNotInFrames…
• (no message - parameterless InvalidOperationException)

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/730561b238536158. Report an issue: GitHub.

Appendix: source

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

            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        protected override bool IsAttachableCore
        {
            get { return _isAttachable; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        protected override bool IsEventCore
        {
            get { return false; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        protected override bool IsDirectiveCore
        {
            get { return false; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        protected override XamlType TargetTypeCore
        {
            get { return null; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        protected override AllowedMemberLocation AllowedLocationCore
        {
            get { return AllowedMemberLocation.Any; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        protected override XamlProperty DependsOnCore
        {
            get { return null; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }

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