实体框架提供跟踪对实体及其关系所做更改的功能，因此在调用上下文的SaveChanges方法时，会对数据库进行正确的更新.这是实体框架的一个关键功能.
更改跟踪跟踪更改，同时将新记录添加到实体收集，修改或删除现有实体.
然后所有更改都由DbContext级别保留.
如果在销毁DbContext对象之前未保存这些跟踪更改，则会丢失这些跟踪更改.
DbChangeTracker类为您提供有关当前的所有信息上下文跟踪的实体.
要按上下文跟踪任何实体，它必须具有主键属性.
在实体框架中，默认情况下启用更改跟踪.您还可以通过将DbContext的AutoDetectChangesEnabled属性设置为false来禁用更改跟踪.如果此属性设置为true，则实体框架维护实体的状态.
using (var context = new UniContextEntities()) {
context.Configuration.AutoDetectChangesEnabled = true;
}让我们看看以下示例，其中从数据库中检索学生及其注册.
class Program {
static void Main(string[] args) {
using (var context = new UniContextEntities()) {
context.Configuration.AutoDetectChangesEnabled = true;
Console.WriteLine("Retrieve Student");
var student = (from s in context.Students where s.FirstMidName ==
"Ali" select s).FirstOrDefault<Student>();
string name = student.FirstMidName + " " + student.LastName;
Console.WriteLine("ID: {0}, Name: {1}", student.ID, name);
Console.WriteLine();
Console.WriteLine("Retrieve all related enrollments");
foreach (var enrollment in student.Enrollments) {
Console.WriteLine("Enrollment ID: {0}, Course ID: {1}",
enrollment.EnrollmentID, enrollment.CourseID);
}
Console.WriteLine();
Console.WriteLine("Context tracking changes of {0} entity.",
context.ChangeTracker.Entries().Count());
var entries = context.ChangeTracker.Entries();
foreach (var entry in entries) {
Console.WriteLine("Entity Name: {0}", entry.Entity.GetType().Name);
Console.WriteLine("Status: {0}", entry.State);
}
Console.ReadKey();
}
}
}编译并执行上述示例时，您将收到以下输出.
Retrieve Student ID: 1, Name: Ali Alexander Retrieve all related enrollments Enrollment ID: 1, Course ID: 1050 Enrollment ID: 2, Course ID: 4022 Enrollment ID: 3, Course ID: 4041 Context tracking changes of 4 entity. Entity Name: Student Status: Unchanged Entity Name: Enrollment Status: Unchanged Entity Name: Enrollment Status: Unchanged Entity Name: Enrollment Status: Unchanged
您可以看到所有数据仅从数据库中检索，这就是为什么所有实体的状态都未更改的原因.
现在让我们看看另一个简单的例子，我们将在其中添加一个注册并从数据库中删除一个学生.以下是添加新注册并删除一名学生的代码.
class Program {
static void Main(string[] args) {
using (var context = new UniContextEntities()) {
context.Configuration.AutoDetectChangesEnabled = true;
Enrollment enr = new Enrollment() {
StudentID = 1, CourseID = 3141
};
Console.WriteLine("Adding New Enrollment");
context.Enrollments.Add(enr);
Console.WriteLine("Delete Student");
var student = (from s in context.Students where s.ID ==
23 select s).SingleOrDefault<Student>();
context.Students.Remove(student);
Console.WriteLine("");
Console.WriteLine("Context tracking changes of {0} entity.",
context.ChangeTracker.Entries().Count());
var entries = context.ChangeTracker.Entries();
foreach (var entry in entries) {
Console.WriteLine("Entity Name: {0}", entry.Entity.GetType().Name);
Console.WriteLine("Status: {0}", entry.State);
}
Console.ReadKey();
}
}
}编译并执行上述示例时，您将收到以下输出./p>
Adding New Enrollment Delete Student Context tracking changes of 2 entity. Entity Name: Enrollment Status: Added Entity Name: Student Status: Deleted
你可以现在看到注册实体的状态设置为已添加，并且学生实体的状态被删除，因为已添加新注册并且从数据库中删除了一个学生.
我们建议为了更好地理解，您可以逐步执行上述示例.
