偶然出现的一个序列化错误，貌似是因为序列化的深度太大导致的。

MongoDB.Bson.BsonSerializationException: Maximum serialization depth exceeded (does the object being serialized have a circular reference?).

at MongoDB.Bson.IO.BsonWriter.WriteStartDocument()

at MongoDB.Bson.IO.BsonBinaryWriter.WriteStartDocument()

at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.SerializeClass(BsonSerializationContext context, BsonSerializationArgs args, TClass document)

at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.Serialize(BsonSerializationContext context, BsonSerializationArgs args, TClass value)

at MongoDB.Bson.Serialization.Serializers.ObjectSerializer.SerializeDiscriminatedValue(BsonSerializationContext context, BsonSerializationArgs args, Object value, Type actualType)

at MongoDB.Bson.Serialization.Serializers.ObjectSerializer.Serialize(BsonSerializationContext context, BsonSerializationArgs args, Object value)

at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Serialize(IBsonSerializer serializer, BsonSerializationContext context, Object value)