JamesNK/Newtonsoft.Json · error · ArgumentException

Accessed JArray values with invalid key value: {0}. Int32 ar

Error message

Accessed JArray values with invalid key value: {0}. Int32 array index expected.

What it means

Thrown by the JArray[object key] getter when the key is neither an int nor (on .NET 6+) a System.Index. JArray is positional, so only integer indices resolve an element; any other key type is invalid.

Source

Thrown at Src/Newtonsoft.Json/Linq/JArray.cs:260

        /// 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));

                switch (key)
                {
                    case int intKey:
                        return GetItem(intKey);
#if NET6_0_OR_GREATER
                    case Index indexKey:
                        return GetItem(indexKey.GetOffset(Count));
#endif
                    default:
                        throw new ArgumentException("Accessed JArray values with invalid key value: {0}. Int32 array index expected.".FormatWith(CultureInfo.InvariantCulture, MiscellaneousUtils.ToString(key)));
                }

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

                switch (key)
                {
                    case int intKey:
                        SetItem(intKey, value);
                        return;
#if NET6_0_OR_GREATER
                    case Index indexKey:
                        SetItem(indexKey.GetOffset(Count), value);
                        return;
#endif
                    default:

View on GitHub (pinned to 4f73e74372)

Solutions

1. Use the strongly-typed int indexer: arr[intIndex].
2. If you need lookup by content, use LINQ (arr.First(t => ...)) or the First/Last properties.
3. If a string key was intended, parse the JSON as JObject instead of JArray.

Example fix

// before
JToken v = myArray[(object)"id"];
// after
JToken v = myArray[0];

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

static bool IsArrayKey(object key) => key is int
#if NET6_0_OR_GREATER
    || key is System.Index
#endif
;

try { var v = arr[key]; }
catch (ArgumentException ex) when (ex.Message.Contains("Int32 array index expected")) {
    // resolve via LINQ or convert key to int
}

When it happens

Trigger: Indexing with a string (arr["name"]), a long, a double, or any non-int object via the object-key indexer.

Common situations: Code written for JObject (string keys) reused on a JArray; passing a dynamic/boxed value whose runtime type is long or byte rather than int.

Related errors

• Set JArray values with invalid key value: {0}. Int32 array i
• Object serialized to {0}. JArray instance expected.
• Accessed JConstructor values with invalid key value: {0}. Ar
• Set JConstructor values with invalid key value: {0}. Argumen
• An ObjectId must be 12 bytes

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/eee0c0445c653278. Report an issue: GitHub.