Btukfyl

How does the Entity Framework ChangeTracker know that a property has changed? The properties have no advanced setter. I would like to know how to track those changes.

If you use the DbContext and DbSets normally, you won't add items to the change tracker. Before you can change any property of an entity, you'll first have to fetch it. Also if you want to delete it, you'll first have to fetch it.

The change tracker holds all fetched items, at least until SaveChanges, but probably until the DbContext is Disposed, if you really want to know, write some test code.

The ChangeTracker also holds all Added items.

The sequence of fetched and added items in the ChangeTracker can be accessed using method Entries or Entries<...>. The returned value is a sequence of DbEntityEntries. Every DbEntityEntry has a State, indicating whether it was Added / Deleted / Modified / Unchanged

After you fetched the item, its state is Unchanged. If you call Delete, the state changes in Deleted. If you add an object the State if Added. The difficult pare is Modified, because you can modify an item without using DbSet:

// Student moves to different school:

int jfkSchoolId = schoolContext.Schools

    .Where(school => school.Name = "J.F. Kennedy School")

    .Select(school => school.Id)

    .FirstOrDefault();

var student = schoolContext.Students.Where(student.Id == 100).SingleOrDefault();

student.SchoolId = jfkSchoolId;

How does the ChangeTracker know you changed the SchoolId?

Luckily, the DbEntityEntry holds the original database values as well as the current values. So when you ask for the state, all it has to do is check if it has been added / deleted / etc. Most states are easy, only if it is Unchanged, the function that fetches the state will have to check all original values with all currentValues, using a default value comparer. If there are differences, the state is marked Changed, so the next time the value comparison is not needed.

You can't undo this: once the State is changed, you can't unchange it:

int originalSchoolId = myStudent.SchoolId;

myStudent.SchoolId = jfxSchoolId;

var state = dbContext.ChangeTracker.Entries<Students>

    .Where(studentEntity => studentEntity.Entity.Id == myStudent.Id)

    .Select(studentEntity => studentEntity.State)

    .SingleOrDefault();

// state equals Changed, because original value was not 0

// student back to original school

myStudent.SchoolId = originalSchoolId;

// ask the state of this student again:    

state = ...

// state is still changed