dotnet/wpf · error · InvalidOperationException

SR.Format(SR.ReadOnlyDesignerCoersionNotAllowed, Name)

Error message

SR.Format(SR.ReadOnlyDesignerCoersionNotAllowed, Name)

What it means

The DesignerCoerceValueCallback property setter throws this InvalidOperationException when the DependencyProperty is read-only. A designer-supplied coercion callback would allow mutating the value of a read-only property, which the key-based security model forbids, so the setter is blocked entirely.

Solutions

1. Do not set DesignerCoerceValueCallback on read-only properties; coerce at the source that writes via the key instead
2. If you own the property and need coercion, re-register it as writable or apply coercion logic inside the code that sets the value through the DependencyPropertyKey
3. Only attach DesignerCoerceValueCallback to writable DPs, guarded by if (!dp.ReadOnly)

Example fix

// before
if (needsCoercion)
    someDp.DesignerCoerceValueCallback = CoerceValue; // throws for read-only DPs
// after
if (needsCoercion && !someDp.ReadOnly)
    someDp.DesignerCoerceValueCallback = CoerceValue;
// for read-only DPs, coerce inside the key-protected setter path instead

if (!dp.ReadOnly)
    dp.DesignerCoerceValueCallback = CoerceValue;
else
    ApplyCoercionAtWriteSite();

static bool AcceptsDesignerCoercion(DependencyProperty dp) => !dp.ReadOnly;

When it happens

Trigger: Assigning dp.DesignerCoerceValueCallback = callback where dp is a read-only property (registered via DependencyPropertyKey), typically from designer tooling or custom property-system extensions.

Common situations: Visual Studio designer or third-party designer extensions trying to install coercion on read-only DPs like IsMouseOver; test harnesses poking at the property-system extension points.

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.Format(SR.ReadOnlyOverrideNotAllowed, Name)
• SR.PropertyNotReadOnly
• SR.ReadOnlyChangeNotAllowed
• Cannot remove signature from read-only file.
• Image_OriginalStreamReadOnly

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/eb75acc4d84aea40. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/DependencyProperty.cs:1125

            // Covered by Synchronized by caller
            return GlobalIndexCount++;
        }

        /// <summary>
        /// This is the callback designers use to participate in the computation of property
        /// values at design time. Eg. Even if the author sets Visibility to Hidden, the designer
        /// wants to coerce the value to Visible at design time so that the element doesn't
        /// disappear from the design surface.
        /// </summary>
        internal CoerceValueCallback DesignerCoerceValueCallback
        {
            get {  return _designerCoerceValueCallback; }
            set
            {
                if (ReadOnly)
                {
                    throw new InvalidOperationException(SR.Format(SR.ReadOnlyDesignerCoersionNotAllowed, Name));
                }

                _designerCoerceValueCallback = value;
            }
        }

        /// <summary> Standard unset value </summary>
        public static readonly object UnsetValue = new NamedObject("DependencyProperty.UnsetValue");

        private string _name;
        private Type _propertyType;
        private Type _ownerType;
        private PropertyMetadata _defaultMetadata;
        private ValidateValueCallback _validateValueCallback;
        private DependencyPropertyKey _readOnlyKey;


        [Flags]

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