Entity Framework Core ChangeTracker

Introduction

ChangeTracker class provides access to change tracking information and operations for entity instances the context is tracking. In EF Core, the DbContext includes the ChangeTracker class, and it starts tracking of all the entities as soon as it is retrieved using DbContext.

The Change Tracker tracks an entity using any of the following states:

• Added
• Unchanged
• Modified
• Deleted
• Detached

Added

The entity is being tracked by the context but does not yet exist in the database. If a new entity or entities are added into the context using Add() method, then it will be marked as Added.

using (var context = new MyContext())
{
    var customer = new Customer
    {
        FirstName = "Thomas",
        LastName = "Hardy",
        Address = "120 Hanover Sq."
    };

    context.Customers.Add(customer);
    foreach (var entry in context.ChangeTracker.Entries())
    {
        Console.WriteLine("Entity: {0},  State: {1} ", entry.Entity.GetType().Name, entry.State.ToString());
    }
}

Deleted

The entity is being tracked by the context and exists in the database. If any entity is removed from the DbContext using the Remove() method, then it will be marked as Deleted.

using (var context = new MyContext())
{
    var customer = context.Customers
        .Where(c => c.FirstName == "Thomas")
        .FirstOrDefault();

    context.Customers.Remove(customer);
    foreach (var entry in context.ChangeTracker.Entries())
    {
        Console.WriteLine("Entity: {0},  State: {1} ", entry.Entity.GetType().Name, entry.State.ToString());
    }
}

Detached

If the entity or entities created or retrieved out of the scope of the current DbContext instance, then it will be marked as Detached.

var customer = new Customer
{
    FirstName = "Elizabeth",
    LastName = "Lincoln",
    Address = "23 Tsawassen Blvd."
};

using (var context = new MyContext())
{
    Console.Write(context.Entry(customer).State);
}

The entities are not being tracked by an existing DbContext instance are also called disconnected entities.

Modified

The entity is being tracked by the context and exists in the database. The entity will be marked as Modified if the value of any property of an entity is changed in the scope of the DbContext.

using (var context = new MyContext())
{
    var customer = context.Customers
        .Where(c => c.FirstName == "Thomas")
        .FirstOrDefault();
    
    customer.LastName = "Anders";
    Console.Write(context.Entry(customer).State);
}

Unchanged

The entity is being tracked by the context and exists in the database. When the entity or entities are retrieved using raw SQL query or LINQ-to-Entities queries will have the Unchanged state.

using (var context = new MyContext())
{
    var customer = context.Customers
        .Where(c => c.FirstName == "Mark")
        .FirstOrDefault();

    Console.Write(context.Entry(customer).State);
}