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：
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[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; }
}
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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：
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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; }
}
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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：
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[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; }
}
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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。