JamesNK/Newtonsoft.Json · error · ArgumentException

Can not convert {0} to Double.

Error message

Can not convert {0} to Double.

What it means

Thrown by the explicit nullable cast operator double?(JToken?) at JToken.cs:764. With nullable:true, JSON null/undefined tokens pass and return null. The error fires when the token is not a JValue 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:764

            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)
        {
            if (value == null)
            {
                return null;
            }

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

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

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

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

View on GitHub (pinned to 4f73e74372)

Solutions

1. Guard with token.Type in {Integer, Float, String, Boolean} before the cast.
2. Use token.Value<double?>() which returns default for incompatible tokens.
3. Inspect the JSON to confirm a scalar number, not a container, is present.
4. Deserialize into a nullable double POCO property.

Example fix

// before
double? v = (double?)token["value"]; // throws when value is a date

// after
double? v = (token["value"]?.Type is JTokenType.Integer or JTokenType.Float)
    ? (double?)token["value"]
    : null;

// Verify the token is a number-compatible scalar (nullable cast tolerates JSON null)
static bool CanToNullableDouble(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 double? SafeDouble(JToken? t) => t switch {
    JValue v when v.Type is JTokenType.Integer or JTokenType.Float
                       or JTokenType.String or JTokenType.Boolean => (double?)t,
    JValue v when v.Type is JTokenType.Null or JTokenType.Undefined => null,
    _ => null
};

try
{
    double? d = (double?)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 JValue or a JObject/JArray to (double?)token; a measurement field whose Type is Date or Object. A JSON null returns null.

Common situations: A numeric measurement field came back as a nested object or date; a Guid/Uri value read as a double; a field re-typed by an upstream service.

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 Decimal.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/792ad89c44f6cdb3. Report an issue: GitHub.