dotnet/wpf · error · NotSupportedException

SR.CantGetWriteonlyProperty

Error message

SR.CantGetWriteonlyProperty

What it means

XamlMemberInvoker.GetValue reads the member's value via its underlying getter. When the member has no getter (write-only property or unknown accessor), NotSupportedException with SR.CantGetWriteonlyProperty (formatted with the member) is thrown, because there is no way to produce a value.

Solutions

  1. Check `invoker.UnderlyingGetter is null` (or member.IsReadPublic / CanRead) before calling GetValue and skip or log instead.
  2. Wrap in try-catch for NotSupportedException if member readability cannot be determined statically.
  3. If read access is genuinely required, add a getter to the property or use the backing field/storage directly.

Example fix

// before
foreach (var m in type.GetAllMembers()) values[m.Name] = m.Invoker.GetValue(obj); // throws on write-only
// after
foreach (var m in type.GetAllMembers())
{
    if (m.Invoker.UnderlyingGetter is not null)
        values[m.Name] = m.Invoker.GetValue(obj);
}
Defensive patterns

Strategy: type-guard

Validate before calling

bool canRead = member.Invoker.UnderlyingGetter is not null;

Type guard

static bool CanRead(XamlMember m) => m.Invoker.UnderlyingGetter is not null;

Try / catch

try { value = invoker.GetValue(instance); }
catch (NotSupportedException) { /* write-only member: skip */ }

Prevention

When it happens

Trigger: Calling invoker.GetValue(instance) where the XamlMember wraps a write-only property (setter only, UnderlyingGetter is null); also invoked by member.UnknownInvoker paths.

Common situations: Generic property-walking code (dumpers, serializers, diff tools) that calls GetValue on every member without checking whether it is readable; attached properties defined with only a setter.

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/d29af14e9be01285. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XamlMemberInvoker.cs:57

        }

        public MethodInfo UnderlyingGetter
        {
            get { return IsUnknown ? null : _member.Getter; }
        }

        public MethodInfo UnderlyingSetter
        {
            get { return IsUnknown ? null : _member.Setter; }
        }

        public virtual object GetValue(object instance)
        {
            ArgumentNullException.ThrowIfNull(instance);
            ThrowIfUnknown();
            if (UnderlyingGetter is null)
            {
                throw new NotSupportedException(SR.Format(SR.CantGetWriteonlyProperty, _member));
            }

            if (UnderlyingGetter.IsStatic)
            {
                return UnderlyingGetter.Invoke(null, new object[] { instance });
            }
            else
            {
                return UnderlyingGetter.Invoke(instance, Array.Empty<object>());
            }
        }

        public virtual void SetValue(object instance, object value)
        {
            ArgumentNullException.ThrowIfNull(instance);
            ThrowIfUnknown();
            if (UnderlyingSetter is null)
            {

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