dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(member))

Error message

ArgumentNullException(nameof(member))

What it means

StaticExtension's string-taking constructor throws ArgumentNullException when the member argument is null. The member string (format Prefix:ClassName.FieldOrPropertyName) is the core state of the extension, so null is rejected immediately at construction. This is eager argument validation consistent with ArrayExtension's constructor guard.

Solutions

1. Pass a non-null member string such as "ClassName.PropertyName" to the constructor.
2. Validate the source string for null before constructing the extension and surface a clearer error at that point.
3. Use the parameterless StaticExtension() constructor plus Member/MemberType assignment if constructing in stages.

Example fix

// before
var ext = new StaticExtension(config["member"]); // config key missing → null
// after
var member = config["member"] ?? throw new InvalidOperationException("Static member not configured");
var ext = new StaticExtension(member);

if (member is null) throw new ArgumentException("member string is required, e.g. 'ClassName.MemberName'", nameof(member));
var ext = new StaticExtension(member);

bool IsValidMemberString(string m) => m is not null;

try { var ext = new StaticExtension(member); }
catch (ArgumentNullException ex) when (ex.ParamName == "member") { /* handle missing member config */ }

When it happens

Trigger: Calling new StaticExtension((string)null) — typically from reflective/generative code that resolves the member string dynamically and gets null.

Common situations: Code generators or XAML infrastructure emitting StaticExtension where the member attribute was absent; config-driven construction where the 'member' key is missing and yields null; tests exercising null-argument contracts.

Related errors

• ArgumentNullException(nameof(arrayType))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))
• ArgumentNullException(nameof(newNamespace))
• ArgumentNullException(nameof(oldNamespace))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c58471639124b0b9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/StaticExtension.cs:37

    {
        private string _member;
        private Type _memberType;

        /// <summary>
        /// Constructor that takes no parameters
        /// </summary>
        public StaticExtension()
        {
        }

        /// <summary>
        /// Constructor that takes the member that this is a static reference to.
        /// This string is of the format Prefix:ClassName.FieldOrPropertyName.
        /// The Prefix is optional, and refers to the XML prefix in a Xaml file.
        /// </summary>
        public StaticExtension(string member)
        {
            _member = member ?? throw new ArgumentNullException(nameof(member));
        }

        /// <summary>
        /// Return an object that should be set on the targetObject's targetProperty
        /// for this markup extension. For a StaticExtension this is a static field
        /// or property value.
        /// </summary>
        /// <param name="serviceProvider">Object that can provide services for the markup extension.</param>
        /// <returns> The object to set on this property.</returns>
        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            if (_member is null)
            {
                throw new InvalidOperationException(SR.MarkupExtensionStaticMember);
            }

            Type type = MemberType;
            string fieldString;

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