What causes "Can not convert {0} to BigInteger." in JamesNK/Newtonsoft.Json?

Guard token["value"]?.Type is in {Integer, Float, String, Boolean, Bytes} before casting.

JamesNK/Newtonsoft.Json · error · ArgumentException

Can not convert {0} to BigInteger.

Error message

Can not convert {0} to BigInteger.

What it means

Thrown by the private helper ToBigInteger(JToken) (exposed via the BigInteger cast operator, guarded by HAVE_BIG_INTEGER) when the token is not a BigInteger-compatible JValue. ArgumentException {0} is the resolved JTokenType; allowed types are BigIntegerTypes ({Integer, Float, String, Comment, Raw, Boolean, Bytes}) — broader than NumberTypes because Bytes is allowed. Non-nullable helper, so a JSON null token throws.

Source

Thrown at Src/Newtonsoft.Json/Linq/JToken.cs:1543

            {
                throw new ArgumentException("Can not convert {0} to Uri.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
            }

            if (v.Value == null)
            {
                return null;
            }

            return (v.Value is Uri uri) ? uri : new Uri(Convert.ToString(v.Value, CultureInfo.InvariantCulture)!);
        }

#if HAVE_BIG_INTEGER
        private static BigInteger ToBigInteger(JToken value)
        {
            JValue? v = EnsureValue(value);
            if (v == null || !ValidateToken(v, BigIntegerTypes, false))
            {
                throw new ArgumentException("Can not convert {0} to BigInteger.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
            }

            return ConvertUtils.ToBigInteger(v.Value!);
        }

        private static BigInteger? ToBigIntegerNullable(JToken value)
        {
            JValue? v = EnsureValue(value);
            if (v == null || !ValidateToken(v, BigIntegerTypes, true))
            {
                throw new ArgumentException("Can not convert {0} to BigInteger.".FormatWith(CultureInfo.InvariantCulture, GetType(value)));
            }

            if (v.Value == null)
            {
                return null;
            }

View on GitHub (pinned to 4f73e74372)

Solutions

Guard token["value"]?.Type is in {Integer, Float, String, Boolean, Bytes} before casting.
Use the nullable BigInteger? cast (ToBigIntegerNullable) so JSON null returns null instead of throwing.
Validate the string parses as an integer with BigInteger.TryParse before casting.
If the field became a GUID/date, change the consuming type rather than forcing a BigInteger cast.

Example fix

// before
BigInteger value = (BigInteger)payload["value"]; // throws on JSON null or GUID

// after
BigInteger? value = payload["value"]?.Type == JTokenType.String
    && System.Numerics.BigInteger.TryParse((string)payload["value"]!, out var bi)
    ? bi : (BigInteger?)null;

Defensive patterns

Strategy: validation

Validate before calling

BigInteger? value = payload["value"]?.Type == JTokenType.String
    && System.Numerics.BigInteger.TryParse((string)payload["value"]!, out var bi)
    ? bi : null;

Type guard

static bool IsBigIntegerLike(JToken? t) =>
    t is JValue v && Array.IndexOf(
        new[] { JTokenType.Integer, JTokenType.Float, JTokenType.String, JTokenType.Boolean, JTokenType.Bytes }, v.Type) >= 0;

Try / catch

BigInteger value;
try { value = (BigInteger)payload["value"]; }
catch (ArgumentException) { value = BigInteger.Zero; }

Prevention

Use the nullable BigInteger? cast (ToBigIntegerNullable) for null-tolerant reads.
BigInteger.TryParse string nodes to also catch non-numeric strings.
BigIntegerTypes includes Bytes — but Date/Guid/Uri/Object tokens still throw.

When it happens

Trigger: Calling (BigInteger)token["value"] when the JValue Type is Date, Guid, Uri, TimeSpan, Null, Object, or Array, or the node is a JObject/JArray (EnsureValue null). A non-numeric string passes ValidateToken then throws FormatException from ConvertUtils.ToBigInteger, not this error.

Common situations: Large-integer field occasionally emitted as a date, GUID, or object; null big-integer in some records; schema drift from a numeric to a GUID/UUID string.

Related errors

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/243c69309f86e813. Report an issue: GitHub.