InvalidEnumArgumentException("value", (int)value…

Error message

InvalidEnumArgumentException("value", (int)value, typeof(XamlWriterMode))

What it means

XamlDesignerSerializationManager.XamlWriterMode validates the assigned value with IsValidXamlWriterMode and throws InvalidEnumArgumentException when the value is not a defined XamlWriterMode member, preventing designers from entering an unknown serialization mode.

Solutions

Only assign values from the XamlWriterMode enum (e.g. XamlWriterMode.Expression)
Validate raw int values against Enum.IsDefined(typeof(XamlWriterMode), v) before casting
Fix config parsing so the stored mode maps to a defined enum member

Example fix

// before
manager.XamlWriterMode = (XamlWriterMode)42;
// after
XamlWriterMode mode;
if (Enum.TryParse("Expression", out mode))
    manager.XamlWriterMode = mode;

Defensive patterns

Strategy: validation

Validate before calling

if (!Enum.IsDefined(typeof(XamlWriterMode), value))
    throw new InvalidEnumArgumentException(nameof(value), (int)value, typeof(XamlWriterMode));

Type guard

bool IsValidXamlWriterMode(XamlWriterMode m) => Enum.IsDefined(typeof(XamlWriterMode), m);

Try / catch

try { manager.XamlWriterMode = mode; }
catch (InvalidEnumArgumentException ex) { /* use a defined XamlWriterMode member */ }

Prevention

Assign only named XamlWriterMode members, never raw ints
Use Enum.IsDefined/Enum.TryParse before casting persisted values
Keep enum types consistent across designer tooling

When it happens

Trigger: Setting XamlDesignerSerializationManager.XamlWriterMode to an undefined XamlWriterMode value, e.g. an invalid cast from int or a value from another enum.

Common situations: Custom designer tooling casting raw ints to XamlWriterMode; storing the mode in config and converting without validation; copy-paste from code using a different enum.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/5a542e517e774c4e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlDesignerSerializationManager.cs:73

        #region Properties

        /// <summary>
        ///     The mode of serialization for 
        ///     all Expressions
        /// </summary>
        public XamlWriterMode XamlWriterMode
        {
            get
            {
                return _xamlWriterMode;
            }

            set
            {
                // Validate Input Arguments
                if (!IsValidXamlWriterMode(value)) 
                {
                    throw new InvalidEnumArgumentException("value", (int)value, typeof(XamlWriterMode));
                }

                _xamlWriterMode = value;
            }
        }

        /// <summary>
        ///     XmlWriter
        /// </summary>
        internal XmlWriter XmlWriter
        {
            get { return _xmlWriter; }
        }

        #endregion Properties

        #region Internal Methods

View on GitHub (pinned to 81131a70a4)