Entity Framework Tutorial Basics（19）：Change Tracking

Change Tracking in Entity Framework:

Here, you will learn how entity framework tracks changes on entities during its life time.

Entity framework supports automatic change tracking of the loaded entities during the life time of the context. DbChangeTracker class gives you all the information about current entities being tracked by the context.

Please note that every entity must have EntityKey (primary key) property in order to be tracked by the context. Entity framework will not add any entity in the conceptual model which does not have an EntityKey property.

The following code snippet shows how context class tracks the entities and changes occurred in it:

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

        Console.WriteLine("Find Student");
        var std1 = ctx.Students.Find(1);

        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("---------------------------------------");
}

Output:

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

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

As you can see in the above sample code snippet and output, context keeps track of entities whenever we retrieve, add, modify or delete any entity. Please notice that context is alive during any of the operations on entities. Context will not keep track if you do any operation on entities out of its scope.