JamesNK/Newtonsoft.Json · error · ArgumentException

Can not convert {0} to Decimal.

Error message

Can not convert {0} to Decimal.

What it means

Thrown by the explicit nullable cast operator decimal?(JToken?) at JToken.cs:736. With nullable:true, JSON null/undefined tokens pass and return null. The ArgumentException is raised when the token is not a JValue (container) or its type is outside NumberTypes = {Integer, Float, String, Comment, Raw, Boolean} — e.g. Date, Guid, Uri, Bytes, Object, Array.

Source

Thrown at Src/Newtonsoft.Json/Linq/JToken.cs:736

        }
#endif

        /// <summary>
        /// Performs an explicit conversion from <see cref="JToken"/> to <see cref="Nullable{T}"/> of <see cref="Decimal"/>.
        /// </summary>
        /// <param name="value">The value.</param>
        /// <returns>The result of the conversion.</returns>
        public static explicit operator decimal?(JToken? value)
        {
            if (value == null)
            {
                return null;
            }

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

#if HAVE_BIG_INTEGER
            if (v.Value is BigInteger integer)
            {
                return (decimal?)integer;
            }
#endif

            return (v.Value != null) ? (decimal?)Convert.ToDecimal(v.Value, CultureInfo.InvariantCulture) : null;
        }

        /// <summary>
        /// Performs an explicit conversion from <see cref="JToken"/> to <see cref="Nullable{T}"/> of <see cref="Double"/>.
        /// </summary>
        /// <param name="value">The value.</param>
        /// <returns>The result of the conversion.</returns>
        public static explicit operator double?(JToken? value)

View on GitHub (pinned to 4f73e74372)

Solutions

1. Check token.Type is Integer, Float, or String before the cast.
2. Use token.Value<decimal?>() to get default for incompatible tokens.
3. Confirm the JSON carries a numeric (or numeric-string) scalar, not a nested object.
4. Deserialize into a POCO with a nullable decimal property for schema enforcement.

Example fix

// before
decimal? amount = (decimal?)token["amount"]; // throws when amount is an object

// after
decimal? amount = (token["amount"]?.Type is JTokenType.Integer or JTokenType.Float or JTokenType.String)
    ? (decimal?)token["amount"]
    : null;

// Verify the token is a number-compatible scalar (nullable cast tolerates JSON null)
static bool CanToNullableDecimal(JToken? t) =>
    t is not JValue v || v.Type is JTokenType.Integer or JTokenType.Float
        or JTokenType.String or JTokenType.Boolean or JTokenType.Null or JTokenType.Undefined;

static decimal? SafeDecimal(JToken? t) => t switch {
    JValue v when v.Type is JTokenType.Integer or JTokenType.Float
                       or JTokenType.String or JTokenType.Boolean => (decimal?)t,
    JValue v when v.Type is JTokenType.Null or JTokenType.Undefined => null,
    _ => null // container or Date/Guid/etc.
};

try
{
    decimal? d = (decimal?)token;
}
catch (ArgumentException ex) when (ex.Message.StartsWith("Can not convert"))
{
    // token is a container or Date/Guid; default to null
}

When it happens

Trigger: Casting a Date/Guid/Uri/Bytes JValue or a JObject/JArray to (decimal?)token; a token whose Type is Date where a currency amount was expected. A JSON null returns null.

Common situations: A monetary field represented as a string-ified object or as a Date; a field re-typed to an object after an API change; reading a Guid identifier as a decimal amount.

Related errors

• Can not convert {0} to Boolean.
• Can not convert {0} to DateTimeOffset.
• Can not convert {0} to Int64.
• Can not convert {0} to DateTime.
• Can not convert {0} to Double.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/0485452fce72caa9. Report an issue: GitHub.