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。


  实体继承的使用

  定义好继承的实体之后，我们就可以使用了。先是自定义一个DataContext吧：

public partial class BBSContext : DataContext
{
public Table<BoardCategory> BoardCategories;
public Table<Board> Boards;
public Table<Topic> Topics;
public BBSContext(string connection) : base(connection) { }
}

  然后，我们来测试一下Linq to sql是否能根据TopicType识别派生类：

BBSContext ctx = new BBSContext("server=xxx;database=BBS;uid=xxx;pwd=xxx");
var query = from t in ctx.Topics select t;
foreach (Topic topic in query)
{
if (topic is NewTopic)
{
NewTopic newtopic = topic as NewTopic;
Response.Write("标题：" + newtopic.TopicTitle + " 类型：" + newtopic.TopicType + "<br/>");
}
else if (topic is Reply)
{
Reply reply = topic as Reply;
Response.Write("标题：" + reply.TopicTitle
+ " 类型：" + reply.TopicType + " 隶属主题：" + reply.ParentTopic + "<br/>");
}
}

  然后我们往Topics表中加一些数据，如下图：

  启动程序得到如下测试结果：

  当然，你也可以在查询句法中直接查询派生实体：

IEnumerable newtopiclist = (from t in ctx.Topics.OfType<NewTopic>() select t).ToList();
newtopics.DataSource = newtopiclist;
IEnumerable replylist = (from t in ctx.Topics.OfType<Reply>() select t).ToList();
replies.DataSource = replylist;
Page.DataBind();