Linq to sql（八）：继承与关系（二）

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; } }        这些实体的定义大家应该很熟悉了。下面，我们再来定义两个实体继承帖子基类，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。