继承可以创建复杂的模型,更好地反映开发人员的思考方式,并减少与这些模型交互所需的工作.与实体一起使用的继承与用于类的继承具有相同的目的,因此开发人员已经知道此功能如何工作的基础知识.
让我们看看下面的示例并创建一个新的控制台应用项目.
第1步 : 通过右键单击项目名称添加ADO.NET实体数据模型,然后选择Add → 新项目...
第2步 : 添加一个实体,并按照模型优先方法一章中提到的所有步骤命名为Person.
步骤3 : 添加一些标量属性,如下图所示.
第4步 : 我们将添加另外两个实体学生和教师,这将继承人员表中的属性.
步骤5 : 现在添加Student实体并从Base类型组合框中选择Person,如下图所示.
第6步 : 同样添加教师实体.
第7步 : 现在将EnrollmentDate标量属性添加到学生实体,将HireDate属性添加到教师实体.
第8步 : 让我们继续生成数据库.
步骤9 : 右键单击设计图面并选择Generate Database from Model ...
第10步 : 要创建新数据库,请单击"新建连接..."将打开以下对话框.单击确定.
步骤11 : 单击完成.这将在项目中添加* .edmx.sql文件.您可以通过打开.sql文件在Visual Studio中执行DDL脚本.现在右键单击并选择执行.
步骤12 : 转到服务器资源管理器,您将看到数据库是使用指定的三个表创建的.
步骤13 : 您还可以看到以下域类也是自动生成的.
public partial class Person { public int ID { get; set; } public string FirstMidName { get; set; } public string LastName { get; set; }}public partial class Student : Person { public System.DateTime EnrollmentDate { get; set; }}public partial class Teacher : Person { public System.DateTime HireDate { get; set; }}
以下是Context类.
public partial class InheritanceModelContainer : DbContext { public InheritanceModelContainer() : base("name = InheritanceModelContainer") {} protected override void OnModelCreating(DbModelBuilder modelBuilder) { throw new UnintentionalCodeFirstException(); } public virtual DbSetPeople { get; set; }}
让我们将一些学生和教师添加到数据库中,然后从数据库中检索它.
class Program { static void Main(string[] args) { using (var context = new InheritanceModelContainer()) { var student = new Student { FirstMidName = "Meredith", LastName = "Alonso", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }; context.People.Add(student); var student1 = new Student { FirstMidName = "Arturo", LastName = "Anand", EnrollmentDate = DateTime.Parse(DateTime.Today.ToString()) }; context.People.Add(student1); var techaer = new Teacher { FirstMidName = "Peggy", LastName = "Justice", HireDate = DateTime.Parse(DateTime.Today.ToString()) }; context.People.Add(techaer); var techaer1 = new Teacher { FirstMidName = "Yan", LastName = "Li", HireDate = DateTime.Parse(DateTime.Today.ToString()) }; context.People.Add(techaer1); context.SaveChanges(); } }}
数据库中添加了学生和教师. NTo检索学生和教师,需要使用 OfType 方法,这将返回与指定部门相关的学生和教师.
Console.WriteLine("All students in database"); Console.WriteLine("");foreach (var student in context.People.OfType()) { string name = student.FirstMidName + " " + student.LastName; Console.WriteLine("ID: {0}, Name: {1}, \tEnrollment Date {2} ", student.ID, name, student.EnrollmentDate.ToString());}Console.WriteLine("");Console.WriteLine("************************************************************ *****");Console.WriteLine("");Console.WriteLine("All teachers in database");Console.WriteLine("");foreach (var teacher in context.People.OfType ()) { string name = teacher.FirstMidName + " " + teacher.LastName; Console.WriteLine("ID: {0}, Name: {1}, \tHireDate {2} ", teacher.ID, name, teacher.HireDate.ToString()); }Console.WriteLine("");Console.WriteLine("************************************************************ *****");Console.ReadKey();
在第一个查询中,当您使用OfType< Student>()时,您将无法访问HireDate,因为HireDate属性是教师实体的一部分,类似的EnrollmentDate属性在您使用OfType< Teacher>()时无法访问
执行上述代码时,您将收到以下输出 :
All students in databaseID: 1, Name: Meredith Alonso, Enrollment Date 10/30/2015 12:00:00 AMID: 2, Name: Arturo Anand, Enrollment Date 10/30/2015 12:00:00 AM***************************************************************** All teachers in databaseID: 3, Name: Peggy Justice, HireDate 10/30/2015 12:00:00 AMID: 4, Name: Yan Li, HireDate 10/30/2015 12:00:00 AM*****************************************************************
我们建议您逐步执行上述示例更好的erstanding.