JamesNK/Newtonsoft.Json · error · JsonSerializationException

Expected Bytes but got {0}.

Error message

Expected Bytes but got {0}.

What it means

Thrown by BsonObjectIdConverter.ReadJson when the JsonReader's current token is not JsonToken.Bytes while deserializing a BsonObjectId. The converter expects a byte array (12 bytes) token; any other token type triggers this JsonSerializationException. The {0} shows the actual token type received.

Source

Thrown at Src/Newtonsoft.Json/Converters/BsonObjectIdConverter.cs:73

            else
            {
                writer.WriteValue(objectId.Value);
            }
        }

        /// <summary>
        /// Reads the JSON representation of the object.
        /// </summary>
        /// <param name="reader">The <see cref="JsonReader"/> to read from.</param>
        /// <param name="objectType">Type of the object.</param>
        /// <param name="existingValue">The existing value of object being read.</param>
        /// <param name="serializer">The calling serializer.</param>
        /// <returns>The object value.</returns>
        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            if (reader.TokenType != JsonToken.Bytes)
            {
                throw new JsonSerializationException("Expected Bytes but got {0}.".FormatWith(CultureInfo.InvariantCulture, reader.TokenType));
            }

            byte[] value = (byte[])reader.Value;

            return new BsonObjectId(value);
        }

        /// <summary>
        /// Determines whether this instance can convert the specified object type.
        /// </summary>
        /// <param name="objectType">Type of the object.</param>
        /// <returns>
        /// 	<c>true</c> if this instance can convert the specified object type; otherwise, <c>false</c>.
        /// </returns>
        public override bool CanConvert(Type objectType)
        {
            return (objectType == typeof(BsonObjectId));
        }

View on GitHub (pinned to 4f73e74372)

Solutions

1. Ensure the token for BsonObjectId properties is JsonToken.Bytes (a 12-byte array).
2. If your data is a hex string, add a converter that reads the string and produces a BsonObjectId, instead of using BsonObjectIdConverter.
3. Migrate away from the obsolete BsonObjectId type to the dedicated Newtonsoft.Json.Bson package's ObjectId.

Example fix

// before: API sends OID as hex string, converter expects bytes
// { "_id": "507f1f77bcf86cd799439011" }
settings.Converters.Add(new BsonObjectIdConverter());
var obj = JsonConvert.DeserializeObject<Foo>(json, settings); // throws

// after: custom converter that handles hex strings
settings.Converters.Add(new HexStringObjectIdConverter());

if (reader.TokenType != JsonToken.Bytes)
    throw new JsonSerializationException($"BsonObjectId expects bytes, got {reader.TokenType}");

static bool IsBytesToken(JsonReader r) => r.TokenType == JsonToken.Bytes;

try { var oid = serializer.Deserialize<BsonObjectId>(reader); }
catch (JsonSerializationException ex) when (ex.Message.Contains("Expected Bytes"))
{
    throw new InvalidDataException("OID field is not a byte array token", ex);
}

When it happens

Trigger: Deserializing JSON where a BsonObjectId-typed property maps to a token that is not a byte array, e.g. a JSON string, integer, or object. Because BsonObjectIdConverter is specific to the obsolete BSON support, this typically happens when JSON produced outside the BSON pipeline (e.g. plain JSON with a string OID) is read by a serializer configured with this converter.

Common situations: Mixing plain-JSON data with the BSON-specific BsonObjectIdConverter. Receiving an OID as a 24-char hex string from an API but the converter expects raw bytes. Legacy code still referencing the obsolete BsonObjectId/BsonObjectIdConverter after migrating the wire format to JSON.

Related errors

• Converter cannot read JSON with the specified existing value
• An ObjectId must be 12 bytes
• Unexpected BsonType value: {0}
• Unable to read beyond the end of the stream.
• Unexpected value type when writing binary: {0}

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/e0a62301f18656ed. Report an issue: GitHub.