Linq有很多值得学习的地方，这里我们主要介绍Linq实体继承的定义，包括介绍Linq to sql支持实体的单表继承等方面。
Linq实体继承的定义
Linq to sql支持实体的单表继承，也就是基类和派生类都存储在一个表中。对于论坛来说，帖子有两种，一种是主题贴，一种是回复帖。那么，我们就先定义帖子基类：
- [Table(Name = "Topics")]
- public class Topic
- {
- [Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true,
IsDbGenerated = true, CanBeNull = false)]- public int TopicID { get; set; }
- [Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false)]
- public int TopicType { get; set; }
- [Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)]
- public string TopicTitle { get; set; }
- [Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)]
- public string TopicContent { get; set; }
- }
这些Linq实体继承的定义大家应该很熟悉了。下面，我们再来定义两个Linq实体继承帖子基类，分别是主题贴和回复贴：
- public class NewTopic : Topic
- {
- public NewTopic()
- {
- base.TopicType = 0;
- }
- }
- public class Reply : Topic
- {
- public Reply()
- {
- base.TopicType = 1;
- }
- [Column(Name = "ParentTopic", DbType = "int", CanBeNull = false)]
- public int ParentTopic { get; set; }
- }
对于主题贴，在数据库中的TopicType就保存为0，而对于回复贴就保存为1。回复贴还有一个相关字段就是回复所属主题贴的TopicID。那么，我们怎么告知Linq to sql在TopicType为0的时候识别为NewTopic，而1则识别为Reply那？只需稍微修改一下前面的Topic实体定义：
- [Table(Name = "Topics")]
- [InheritanceMapping(Code = 0, Type = typeof(NewTopic), IsDefault = true)]
- [InheritanceMapping(Code = 1, Type = typeof(Reply))]
- public class Topic
- {
- [Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true,
IsDbGenerated = true, CanBeNull = false)]- public int TopicID { get; set; }
- [Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false,
IsDiscriminator = true)]- public int TopicType { get; set; }
- [Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)]
- public string TopicTitle { get; set; }
- [Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)]
- public string TopicContent { get; set; }
- }
为类加了InheritanceMapping特性定义，0的时候类型就是NewTopic，1的时候就是Reply。并且为TopicType字段上的特性中加了IsDiscriminator = true，告知Linq to sql这个字段就是用于分类的字段。
【编辑推荐】
