dotnet/wpf · error · InvalidOperationException

SR.MustNotCallSetter

Error message

SR.MustNotCallSetter

What it means

XaslMemberReference overrides NameCore and IsPublicCore with getters backed by _name/Ref; their setters throw InvalidOperationException(SR.MustNotCallSetter). The member reference's name and visibility are derived from its underlying reflection reference at construction and are immutable, so the setters are contract stubs only.

Solutions

1. Do not assign Core properties; read them only
2. If a different name/visibility is needed, build a new member reference with the correct underlying Ref
3. Guard bulk-copy code against setters that throw

Example fix

// before
memberRef.NameCore = "CorrectName";
// after
var memberRef = new XaslMemberReference(ownerType, "CorrectName"); // correct at construction

if (memberRef.NameCore !== desiredName) { memberRef = new XaslMemberReference(ownerType, desiredName); } // construct, don't assign

function hasName(m: XamlMember, name: string): boolean { return m.NameCore === name; }

try { /* name-dependent logic */ } catch (InvalidOperationException ex) when (ex.Message.Contains("MustNotCallSetter")) { // rebuild the member reference with the correct name }

When it happens

Trigger: Assigning to NameCore or IsPublicCore on a XaslMemberReference instance, e.g. from schema code that resolves a reference and tries to correct its name or visibility in place.

Common situations: Custom IXamlSchemaProvider code fixing up member names after lookup; metadata transformation pipelines that mutate resolved 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

• SR.CantSetReadonlyProperty
• SR.MustNotCallSetter
• Animation_Invalid_DefaultValue
• Cannot remove signature from read-only file.
• Image_EncoderNoColorContext

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/3c1a40ea4dc7fc67. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XaslMemberReference.cs:76

                    // and we do want some sort of error here if it fails.
                    _ref = (XaslMember)xamlMember;
                }
                return _ref;
            }
        }

        public override String BoundName
        {
            get { return "XASL member " + _name; }
        }

        public override bool IsImplicit { get { return false; } }
        public override bool IsUnknown { get { return false; } }

        protected override String NameCore
        {
            get { return _name; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter);  }
        }

        protected override bool IsPublicCore
        {
            get { return Ref.IsPublic; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter);  }
        }

        protected override bool IsBrowsableCore
        {
            get { return Ref.IsBrowsable; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        protected override bool IsObsoleteCore
        {
            get { return Ref.IsObsolete; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }

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