JamesNK/Newtonsoft.Json · error · ArgumentException

MemberInfo '{0}' has index parameters

Error message

MemberInfo '{0}' has index parameters

What it means

Thrown by ReflectionUtils.GetMemberValue when PropertyInfo.GetValue throws TargetParameterCountException because the property requires index parameters (an indexer). It wraps the inner TargetParameterCountException. Indexed properties such as C# `this[int]` cannot be read without supplying an index.

Source

Thrown at Src/Newtonsoft.Json/Utilities/ReflectionUtils.cs:546

        /// <param name="target">The target object.</param>
        /// <returns>The member's value on the object.</returns>
        public static object? GetMemberValue(MemberInfo member, object target)
        {
            ValidationUtils.ArgumentNotNull(member, nameof(member));
            ValidationUtils.ArgumentNotNull(target, nameof(target));

            switch (member.MemberType())
            {
                case MemberTypes.Field:
                    return ((FieldInfo)member).GetValue(target);
                case MemberTypes.Property:
                    try
                    {
                        return ((PropertyInfo)member).GetValue(target, null);
                    }
                    catch (TargetParameterCountException e)
                    {
                        throw new ArgumentException("MemberInfo '{0}' has index parameters".FormatWith(CultureInfo.InvariantCulture, member.Name), e);
                    }
                default:
                    throw new ArgumentException("MemberInfo '{0}' is not of type FieldInfo or PropertyInfo".FormatWith(CultureInfo.InvariantCulture, member.Name), nameof(member));
            }
        }

        /// <summary>
        /// Sets the member's value on the target object.
        /// </summary>
        /// <param name="member">The member.</param>
        /// <param name="target">The target.</param>
        /// <param name="value">The value.</param>
        public static void SetMemberValue(MemberInfo member, object target, object? value)
        {
            ValidationUtils.ArgumentNotNull(member, nameof(member));
            ValidationUtils.ArgumentNotNull(target, nameof(target));

            switch (member.MemberType())

View on GitHub (pinned to 4f73e74372)

Solutions

1. Skip indexed properties: check propertyInfo.GetIndexParameters().Length == 0 before reading.
2. If you must read an indexer, call PropertyInfo.GetValue directly with the index arguments.
3. Annotate indexers with [JsonIgnore] to exclude them from serialization.

Example fix

// before
var value = ReflectionUtils.GetMemberValue(indexerProperty, target);

// after
if (((PropertyInfo)indexerProperty).GetIndexParameters().Length == 0)
    var value = ReflectionUtils.GetMemberValue(indexerProperty, target);

if (member is PropertyInfo p && p.GetIndexParameters().Length > 0)
    throw new ArgumentException($"{p.Name} is an indexer; use GetValue with index args.");

static bool IsIndexer(PropertyInfo p) => p.GetIndexParameters().Length > 0;

When it happens

Trigger: Calling GetMemberValue on a PropertyInfo whose GetIndexParameters().Length > 0 (an indexer). Internally can occur when a serializer or utility walks all properties including indexers.

Common situations: Custom serializers iterating PropertyInfo collections that include indexers, types with indexers mistaken for ordinary properties, or dictionary-like wrapper types exposing `this[object]`.

Related errors

• Property '{0}' does not have a getter.
• Property does not have a getter.
• Property does not have a setter.
• Could not create getter for {0}. ByRef return values are not
• Accessed JArray values with invalid key value: {0}. Int32 ar

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/ff879669b14d75dd. Report an issue: GitHub.