在前面的章节中，您学习了三种不同的方法来定义实体数据模型.
其中两个，数据库优先和模型优先，取决于实体框架设计师与代码生成的结合.
第三，Code First，让你跳过一个视觉设计师，然后写您自己的代码.
无论您选择哪条路径，您最终都会获得域类，并且一个或多个Entity Framework DbContext类允许您检索和保留与这些类相关的数据.
应用程序中的DbContext API用作类和数据库之间的桥梁. DbContext是实体框架中最重要的类之一.
它可以表达和执行查询.
它从数据库中获取查询结果并将它们转换为模型类的实例.
它可以跟踪实体的更改，包括添加和删除，然后触发创建按需发送到数据库的插入，更新和删除语句.
以下是我们将在本章中执行不同操作的域广告上下文类.这是我们在chapater，Database First Approach中创建的相同示例.
using System;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.Core.Objects;
using System.Linq;
namespace DatabaseFirstDemo {
public partial class UniContextEntities : DbContext {
public UniContextEntities(): base("name = UniContextEntities") {}
protected override void OnModelCreating(DbModelBuilder modelBuilder) {
throw new UnintentionalCodeFirstException();
}
public virtual DbSet<Course> Courses { get; set; }
public virtual DbSet<Enrollment> Enrollments { get; set; }
public virtual DbSet<Student> Students { get; set; }
}
}namespace DatabaseFirstDemo {
using System;
using System.Collections.Generic;
public partial class Course {
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage",
"CA2214:DoNotCallOverridableMethodsInConstructors")]
public Course() {
this.Enrollments = new HashSet<Enrollment>();
}
public int CourseID { get; set; }
public string Title { get; set; }
public int Credits { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage",
"CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection<Enrollment> Enrollments { get; set; }
}
}namespace DatabaseFirstDemo {
using System;
using System.Collections.Generic;
public partial class Student {
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage",
"CA2214:DoNotCallOverridableMethodsInConstructors")]
public Student() {
this.Enrollments = new HashSet<Enrollment>();
}
public int ID { get; set; }
public string LastName { get; set; }
public string FirstMidName { get; set; }
public System.DateTime EnrollmentDate { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage",
"CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection<Enrollment> Enrollments { get; set; }
}
}namespace DatabaseFirstDemo {
using System;
using System.Collections.Generic;
public partial class Enrollment {
public int EnrollmentID { get; set; }
public int CourseID { get; set; }
public int StudentID { get; set; }
public Nullable<int> Grade { get; set; }
public virtual Course Course { get; set; }
public virtual Student Student { get; set; }
}
}使用Entity Framework添加新对象是就像构造对象的新实例并使用DbSet上的Add方法注册它一样简单.以下代码允许您将新学生添加到数据库中.
class Program {
static void Main(string[] args) {
var newStudent = new Student();
//set student name
newStudent.FirstMidName = "Bill";
newStudent.LastName = "Gates";
newStudent.EnrollmentDate = DateTime.Parse("2015-10-21");
newStudent.ID = 100;
//create DBContext object
using (var dbCtx = new UniContextEntities()) {
//Add Student object into Students DBset
dbCtx.Students.Add(newStudent);
// call SaveChanges method to save student into database
dbCtx.SaveChanges();
}
}
}更改现有对象是就像更新分配给您想要更改的属性的值并调用SaveChanges一样简单.例如，以下代码用于将Ali的姓氏从Khan更改为Aslam.
using (var context = new UniContextEntities()) {
var student = (from d in context.Students where d.FirstMidName == "Ali" select d).Single();
student.LastName = "Aslam";
context.SaveChanges();
}要使用Entity Framework删除实体，请使用Remove方法DbSet.删除现有和新添加的实体的工作.在已添加但尚未保存到数据库的实体上调用Remove将取消添加实体.该实体将从更改跟踪器中删除，并且不再由DbContext跟踪.在正在进行更改跟踪的现有实体上调用Remove将在下次调用SaveChanges时注册该实体以进行删除.以下示例是一个代码，其中学生从名为Ali的数据库中删除.
using (var context = new UniContextEntities()) {
var bay = (from d in context.Students where d.FirstMidName == "Ali" select d).Single();
context.Students.Remove(bay);
context.SaveChanges();
}从数据库中读取现有数据非常简单.以下是代码，其中检索Student表中的所有数据，然后将按字母顺序显示学生的名字和姓氏.
using (var db = new UniContextEntities()) {
var query = from b in db.Students orderby b.FirstMidName select b;
Console.WriteLine("All All student in the database:");
foreach (var item in query) {
Console.WriteLine(item.FirstMidName +" "+ item.LastName);
}
Console.WriteLine("Press any key to exit...");
Console.ReadKey();
}
