dotnet/wpf · error · InvalidOperationException

SR.MustNotCallSetter

Error message

SR.MustNotCallSetter

What it means

XaslMember (a legacy internal XAML member type) declares `new public XamlType DeclaringType` whose setter unconditionally throws InvalidOperationException('MustNotCallSetter'). The property is read-only in spirit — declaring type is fixed at construction — and mutating it through the shadowing setter is explicitly disallowed by the base-member contract.

Solutions

  1. Set the declaring type at construction time (constructor argument) rather than via the property.
  2. Remove the mutation — a member's declaring type is fixed identity; create a new XaslMember bound to the correct type instead.
  3. Catch InvalidOperationException if assignment comes from generic property-setting code and skip that property.

Example fix

// before
member.DeclaringType = newType;
// after
var rebuilt = new XaslMember(member.Name, newType);
Defensive patterns

Strategy: try-catch

Validate before calling

// DeclaringType has no usable setter; never assign it.
bool isMutable = typeof(XamlMember).GetProperty("DeclaringType")!.CanWrite; // false by design

Type guard

static bool CanSetDeclaringType(object m) => m is not XaslMember;

Try / catch

try { prop.SetValue(member, newType); } catch (InvalidOperationException) { /* read-only member property */ }

Prevention

When it happens

Trigger: Assigning to DeclaringType on a XaslMember instance, e.g. myXaslMember.DeclaringType = someXamlType; — typically code written against the base XamlMember API surface that assumes the setter exists and works.

Common situations: Migration or tooling code that re-parents members between types at runtime; generic reflection/serialization code that sets properties via PropertyInfo.SetValue; porting older code that used a mutable DeclaringType on a different member implementation.

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/e1fdcf2186c50f2e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XaslMember.cs:73

        [DefaultValue(true)]
        new public bool IsBrowsable
        {
            get { return IsBrowsableCore; }
            set { IsBrowsableCore = value; }
        }

        [DefaultValue(false)]
        new public bool IsObsolete
        {
            get { return IsObsoleteCore; }
            set { IsObsoleteCore = value; }
        }

        new public XamlType DeclaringType
        {
            get { return DeclaringTypeCore; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }

        // Should the default be "x:String"?
        // We don't currently have a type converter to support: <XaslProperty Name="blah" Type="x:String" />
        [TypeConverter(typeof(XaslTypeReferenceConverter))]
        new public XamlType Type
        {
            get { return TypeCore; }
            set { TypeCore = value; }
        }

        [DefaultValue(null)]
        new public XamlTextSyntax TextSyntax
        {
            get { return TextSyntaxCore; }
            set { TextSyntaxCore = value; }
        }

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