Linq to sql（八）：继承与关系（三）

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：

       newtopic和replies是两个GridView控件，执行效果如下图：

       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);

        ctx.Topics.Remove(rpl);

        ctx.SubmitChanges();