Entity Framework入门教程（15)---DbContext追踪实体状态改变

这一节介绍DbContext追踪实体的变化。EF支持DbContext在其生命周期中自动追踪加载的实体。我们可以通过DbChangeTracker类获取DbContext追踪的所有实体的变化。

注意每个实体必须有主键值才能被上下文追踪。概念模型中的实例如果没有主键值，DbContext不会对其执行追踪。

下边的代码显示了DbContext追踪实体状态变化过程

static void Main(string[] args)
{
    using (var ctx = new SchoolDBEntities())
    {

        Console.WriteLine("Find Student");
        var std1 = ctx.Students.Find(1);
　　　　　//ctx.ChangeTracker.Enties()方法获取所有追踪实体的DbEntityEntry对象
        Console.WriteLine("Context tracking changes of {0} entity.", ctx.ChangeTracker.Entries().Count());

        DisplayTrackedEntities(ctx.ChangeTracker);

        Console.WriteLine("Find Standard");

        var standard = ctx.Standards.Find(1);

        Console.WriteLine("Context tracking changes of {0} entities.", ctx.ChangeTracker.Entries().Count());
        Console.WriteLine("");
        Console.WriteLine("Editing Standard");
                
        standard.StandardName = "Edited name";
        DisplayTrackedEntities(ctx.ChangeTracker);


        Teacher tchr = new Teacher() { TeacherName = "new teacher" };
        Console.WriteLine("Adding New Teacher");

        ctx.Teachers.Add(tchr);
        Console.WriteLine("");
        Console.WriteLine("Context tracking changes of {0} entities.", ctx.ChangeTracker.Entries().Count());
        DisplayTrackedEntities(ctx.ChangeTracker);

        Console.WriteLine("Remove Student");
        Console.WriteLine("");

        ctx.Students.Remove(std1);
        DisplayTrackedEntities(ctx.ChangeTracker);
    }
}

private static void DisplayTrackedEntities(DbChangeTracker changeTracker)
{
    Console.WriteLine("");

    var entries = changeTracker.Entries();
    foreach (var entry in entries)
    {
        Console.WriteLine("Entity Name: {0}", entry.Entity.GetType().FullName);
        Console.WriteLine("Status: {0}", entry.State);
    }
    Console.WriteLine("");
    Console.WriteLine("

输出为：

Find Student 
Context tracking changes of 1 entity.

Entity Name: EFTutorials.Student
Status: Unchanged

---------------------------------------
Find Standard
Context tracking changes of 2 entities.

Editing Standard

Entity Name: EFTutorials.Standard
Status: Modified
Entity Name: EFTutorials.Student
Status: Unchanged

---------------------------------------
Adding New Teacher

Context tracking changes of 3 entities.

Entity Name: EFTutorials.Teacher
Status: Added
Entity Name: EFTutorials.Standard
Status: Modified
Entity Name: EFTutorials.Student
Status: Unchanged

---------------------------------------
Remove Student

Entity Name: EFTutorials.Teacher
Status: Added
Entity Name: EFTutorials.Standard
Status: Modified
Entity Name: EFTutorials.Student
Status: Deleted

---------------------------------------

通过上边的栗子可以：无论我们进行是哪种操作（获取，添加，修改，删除），DbContext（在上下文的生命周期内）都会持续追踪实体的状态的改变。

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