JamesNK/Newtonsoft.Json · error · JsonException

Index {0} outside the bounds of JConstructor.

Error message

Index {0} outside the bounds of JConstructor.

What it means

Thrown by PathFilter.GetTokenIndex when an index applied to a JConstructor token is greater than or equal to the constructor's Count, but only when JsonSelectSettings.ErrorWhenNoMatch is true. JConstructor is the JSON construct for non-object/array container types (e.g. constructor syntax like new Date(...)).

Source

Thrown at Src/Newtonsoft.Json/Linq/JsonPath/PathFilter.cs:33

                if (a.Count <= index)
                {
                    if (settings?.ErrorWhenNoMatch ?? false)
                    {
                        throw new JsonException("Index {0} outside the bounds of JArray.".FormatWith(CultureInfo.InvariantCulture, index));
                    }

                    return null;
                }

                return a[index];
            }
            else if (t is JConstructor c)
            {
                if (c.Count <= index)
                {
                    if (settings?.ErrorWhenNoMatch ?? false)
                    {
                        throw new JsonException("Index {0} outside the bounds of JConstructor.".FormatWith(CultureInfo.InvariantCulture, index));
                    }

                    return null;
                }

                return c[index];
            }
            else
            {
                if (settings?.ErrorWhenNoMatch ?? false)
                {
                    throw new JsonException("Index {0} not valid on {1}.".FormatWith(CultureInfo.InvariantCulture, index, t.GetType().Name));
                }

                return null;
            }
        }

View on GitHub (pinned to 4f73e74372)

Solutions

1. Verify the token type (token.Type == JTokenType.Constructor) and its Count before indexing.
2. Drop ErrorWhenNoMatch to make out-of-range indices return null silently.
3. Adjust the index to a valid position or use property/scan filters instead.

Example fix

// before
token.SelectTokens("$[2]", new JsonSelectSettings { ErrorWhenNoMatch = true });
// after
var ctor = (JConstructor)token;
if (ctor.Count > 2)
{
    token.SelectTokens("$[2]", new JsonSelectSettings { ErrorWhenNoMatch = true });
}

static bool CanIndex(JToken token, int index)
{
    return token is JConstructor c && index >= 0 && index < c.Count;
}

bool IsIndexableConstructor(JToken t) => t is JConstructor c && c.Count > 0;

try { token.SelectTokens(path, settings).ToList(); }
catch (JsonException ex) when (ex.Message.Contains("outside the bounds of JConstructor"))
{ /* constructor argument index out of range */ }

When it happens

Trigger: Running an index filter over a JConstructor token with ErrorWhenNoMatch=true where the index is out of range. Example: token.SelectTokens("$[1]", new JsonSelectSettings { ErrorWhenNoMatch = true }) when token is a JConstructor with a single argument.

Common situations: Parsing JSON that uses constructor syntax and then running a positional index on it; payloads whose constructor arity changed; assuming the token is an array when it is actually a constructor.

Related errors

• Index {0} outside the bounds of JArray.
• Path ended with open query.
• Unknown escape character: \
• Path ended with an open string.
• Path ended with an open regex.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/be9e1ff83df16579. Report an issue: GitHub.