Tracking vs. No-Tracking

Remarks#

Tracking behavior controls whether or not Entity Framework will keep information about an entity instance in its change tracker. If an entity is tracked, any changes detected in the entity will be persisted to the database during SaveChanges().

Tracking queries

By default, queries that return entity types are tracking
This means you can make changes to those entity instances and have those changes persisted by SaveChanges()

Example :

The change to the book rating will be detected and persisted to the database during SaveChanges().

using (var context = new BookContext()) { var book = context.Books.FirstOrDefault(b => b.BookId == 1); book.Rating = 5; context.SaveChanges(); }

No-tracking queries

No tracking queries are useful when the results are used in a read-only scenario
They are quicker to execute because there is no need to setup change tracking information

Example :

using (var context = new BookContext())
{
    var books = context.Books.AsNoTracking().ToList();
}

With EF Core 1.0 you are also able to change the default tracking behavior at the context instance level.

Example :

using (var context = new BookContext())
{
    context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;

    var books = context.Books.ToList();
}

Tracking and projections

Even if the result type of the query isn’t an entity type, if the result contains entity types they will still be tracked by default

Example :

In the following query, which returns an anonymous type, the instances of Book in the result set will be tracked

 using (var context = new BookContext())
 {
    var book = context.Books.Select(b => new { Book = b, Authors = b.Authors.Count() });
 }

If the result set does not contain any entity types, then no tracking is performed

Example :

In the following query, which returns an anonymous type with some of the values from the entity (but no instances of the actual entity type), there is no tracking performed.
```
using (var context = new BookContext())
{
   var book = context.Books.Select(b => new { Id = b.BookId, PublishedDate = b.Date });
}
```