How change tracking works in Entity Framework

Question

Given the following code, how does EF/DbContext knows about the change made to the customer object:

class Program
{
    static void Main()
    {
        using(var shopContext = new ShopContext())
        {
            var customer = shopContext.Customers.Find(7);

            customer.City = "Marion";

            customer.State = "Indiana";

            shopContext.SaveChanges();
        }
    }
}

public class ShopContext : DbContext
{
    public DbSet<Customer> Customers { get; set; }
}

public class Customer
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string City { get; set; }
    public string State { get; set; }
}

Thank you

Answer 1

When you load the entity from the context it keeps an additional data structure - let's call it entry. The entry contains two set of values - original values and current values. When you execute the SaveChanges operation EF goes through your customer entities and updates current values in the entry so that they match with the real state of your entity - this operation is called detecting changes. During SQL command generation EF will compare current and original values and build an SQL update statement to modify changed values in the database. This operation is called snapshot change tracking - EF keeps a snap shot in the entry.

There is an alternative called dynamic change tracking which will modify the current value in the entry at the same time you assign the value to your entity's property. Dynamic change tracking has specific requirements (like all of your properties in the entity must be virtual) because it must wrap your class to a dynamic proxy at runtime. This used to be the preferred way but due to some performance issues in complex scenarios, snapshot change tracking is currently supposed to be used as default.