JamesNK/Newtonsoft.Json · error · ArgumentOutOfRangeException

arrayIndex is less than 0.

Error message

arrayIndex is less than 0.

What it means

CopyItemsTo throws ArgumentOutOfRangeException when arrayIndex is negative. The destination offset must be a valid non-negative position in the target array.

Source

Thrown at Src/Newtonsoft.Json/Linq/JContainer.cs:597

            int index = IndexOfItem(existing);
            SetItem(index, replacement);
        }

        internal virtual bool ContainsItem(JToken? item)
        {
            return (IndexOfItem(item) != -1);
        }

        internal virtual void CopyItemsTo(Array array, int arrayIndex)
        {
            if (array == null)
            {
                throw new ArgumentNullException(nameof(array));
            }
            if (arrayIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(arrayIndex), "arrayIndex is less than 0.");
            }
            if (arrayIndex >= array.Length && arrayIndex != 0)
            {
                throw new ArgumentException("arrayIndex is equal to or greater than the length of array.");
            }
            if (Count > array.Length - arrayIndex)
            {
                throw new ArgumentException("The number of elements in the source JObject is greater than the available space from arrayIndex to the end of the destination array.");
            }

            int index = 0;
            foreach (JToken token in ChildrenTokens)
            {
                array.SetValue(token, arrayIndex + index);
                index++;
            }
        }

View on GitHub (pinned to 4f73e74372)

Solutions

  1. Validate arrayIndex >= 0 before calling CopyTo.
  2. Use 0 when you want to copy from the start.
  3. Clamp computed offsets to a minimum of 0.

Example fix

// before
arr.CopyTo(dest, offset - 1);
// after
arr.CopyTo(dest, Math.Max(0, offset - 1));
Defensive patterns

Strategy: validation

Validate before calling

if (arrayIndex < 0) throw new ArgumentOutOfRangeException(nameof(arrayIndex));
container.CopyTo(dest, arrayIndex);

Try / catch

try { container.CopyTo(dest, arrayIndex); }
catch (ArgumentOutOfRangeException ex) when (ex.ParamName == "arrayIndex") {
    container.CopyTo(dest, 0);
}

Prevention

When it happens

Trigger: jArray.CopyTo(arr, -1); a computed offset that underflows below zero.

Common situations: Off-by-one arithmetic on the offset; passing an index derived from an unfound position.

Related errors

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/9954bc139e45903a. Report an issue: GitHub.