Linq to sql：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Linqto 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 inquery)

                {

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 asReply;

                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

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：

ewtopic 和replies 是两个GridView 控件，

再来看看如何进行增删操作：

NewTopic nt = new NewTopic() { TopicTitle = "还是新主题",TopicContent = "还

是新主题" };

Reply rpl = new Reply() { TopicTitle = "还是新回复",TopicContent = "还是新回复

", ParentTopic = 4 };

       ctx.Topics.Add(nt);

       ctx.Topics.Add(rpl);

       ctx.SubmitChanges();

       rpl = ctx.Topics.OfType<Reply>().Single(reply => reply.TopicID ==8);