JamesNK/Newtonsoft.Json · error · ArgumentException

Accessed JConstructor values with invalid key value: {0}. Ar

Error message

Accessed JConstructor values with invalid key value: {0}. Argument position index expected.

What it means

Thrown by the JConstructor[object key] getter when the key is not an int. JConstructor arguments are positional (unlike JProperty), so only integer argument positions are accepted.

Source

Thrown at Src/Newtonsoft.Json/Linq/JConstructor.cs:194

                _values[i].WriteTo(writer, converters);
            }

            writer.WriteEndConstructor();
        }

        /// <summary>
        /// Gets the <see cref="JToken"/> with the specified key.
        /// </summary>
        /// <value>The <see cref="JToken"/> with the specified key.</value>
        public override JToken? this[object key]
        {
            get
            {
                ValidationUtils.ArgumentNotNull(key, nameof(key));

                if (!(key is int i))
                {
                    throw new ArgumentException("Accessed JConstructor values with invalid key value: {0}. Argument position index expected.".FormatWith(CultureInfo.InvariantCulture, MiscellaneousUtils.ToString(key)));
                }

                return GetItem(i);
            }
            set
            {
                ValidationUtils.ArgumentNotNull(key, nameof(key));

                if (!(key is int i))
                {
                    throw new ArgumentException("Set JConstructor values with invalid key value: {0}. Argument position index expected.".FormatWith(CultureInfo.InvariantCulture, MiscellaneousUtils.ToString(key)));
                }

                SetItem(i, value);
            }
        }

        internal override int GetDeepHashCode()

View on GitHub (pinned to 4f73e74372)

Solutions

Use an integer argument position: ctor[0].
Iterate Children() to find a specific argument by value.
Re-check whether you actually need a JObject for named access.

Example fix

// before
JToken a = myCtor[(object)"x"];
// after
JToken a = myCtor[0];

Defensive patterns

Strategy: type-guard

Validate before calling

if (key is int i) { JToken v = ctor[i]; }
else throw new InvalidOperationException("JConstructor requires an int argument index.");

Type guard

static bool IsCtorKey(object key) => key is int;

Try / catch

try { var v = ctor[key]; }
catch (ArgumentException ex) when (ex.Message.Contains("Argument position index expected")) {
    // fall back to integer index
}

Prevention

Always use integer positions for JConstructor arguments.
Remember JConstructor is positional, not keyed.
Wrap dynamic keys in an int-coercion check.

When it happens

Trigger: Indexing a JConstructor with a string or any non-int key on the read side.

Common situations: Assuming JConstructor supports named lookup like JObject; dynamic key dispatch.

Related errors

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/3da9929ab8a380d2. Report an issue: GitHub.