public enum ContentType
    {
        TXT,

        MD,

        HTML
    }

    [Table("card")]
    public class Card: BaseEntityUInt64
    {
        [Column("uid")]
        public ulong UID { get; set; }

        [Column("create_at")]
        public DateTime CreateAt { get; set; }

        [Column("title")]
        public string Title { get; set; }

        [Column("content")]
        public string Content { get; set; }

        [Column("content_type")]
        public ContentType ContentType { get; set; }

        public Card()
        {
            this.Title = string.Empty;
            this.Content = string.Empty;
            this.ContentType = ContentType.TXT;
        }
    }

CREATE TABLE `card` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'ID服务中心获取ID，以便将来分库',
  `uid` bigint(20) DEFAULT NULL,
  `create_at` datetime DEFAULT NULL,
  `title` varchar(128) DEFAULT NULL,
  `content` text,
  `content_type` enum('TXT','MD','HTML') NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id_UNIQUE` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=11 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

    public class EnumTypeHandler<T> : SqlMapper.TypeHandler<T> where T : struct, IConvertible
    {
        static EnumTypeHandler()
        {
            if (!typeof(T).IsEnum)
            {
                throw new ArgumentException("T must be an enumeration type");
            }
        }

        public override T Parse(object value)
        {
            return (T)Enum.Parse(typeof(T), Convert.ToString(value));
        }

        public override void SetValue(IDbDataParameter parameter, T value)
        {
            parameter.Value = value.ToString();
            parameter.DbType = DbType.AnsiString;
        }
    }