Change Tracking in EF Core

What is Change Tracking?

Change tracking is the mechanism by which EF Core keeps track of changes made to entities so it can generate appropriate SQL commands when SaveChanges() is called.

Entity States

public enum EntityState
{
    Detached,   // Not tracked
    Unchanged,  // Tracked, not modified
    Added,      // New entity, will be inserted
    Modified,   // Tracked and modified
    Deleted     // Marked for deletion
}

Checking Entity State

var product = context.Products.Find(1);
var state = context.Entry(product).State;

Console.WriteLine(state); // Unchanged

product.Price = 99.99m;
state = context.Entry(product).State;
Console.WriteLine(state); // Modified

Change Tracker

// Access all tracked entities
var entries = context.ChangeTracker.Entries();

// Get modified entities
var modifiedEntries = context.ChangeTracker.Entries()
    .Where(e => e.State == EntityState.Modified);

// Detect changes manually
context.ChangeTracker.DetectChanges();

// Clear all tracked entities
context.ChangeTracker.Clear();

Tracking Behavior

Default Tracking

// Entities are tracked by default
var products = context.Products.ToList();
products[0].Price = 99.99m;
await context.SaveChangesAsync(); // Updates tracked

No Tracking

// Read-only queries
var products = context.Products
    .AsNoTracking()
    .ToList();

// Changes won't be saved
products[0].Price = 99.99m;
await context.SaveChangesAsync(); // No update

No Tracking with Identity Resolution

var products = context.Products
    .AsNoTrackingWithIdentityResolution()
    .Include(p => p.Category)
    .ToList();

Modifying Tracked Entities

// Modify tracked entity
var product = await context.Products.FindAsync(1);
product.Price = 99.99m;
await context.SaveChangesAsync();

// Modify specific properties
var product = await context.Products.FindAsync(1);
context.Entry(product).Property(p => p.Price).CurrentValue = 99.99m;
await context.SaveChangesAsync();

Attaching Entities

// Attach disconnected entity
var product = new Product { Id = 1, Name = "Updated", Price = 99.99m };
context.Products.Attach(product);
context.Entry(product).State = EntityState.Modified;
await context.SaveChangesAsync();

// Update disconnected entity
context.Products.Update(product);
await context.SaveChangesAsync();

Change Tracking Events

public class ApplicationDbContext : DbContext
{
    public ApplicationDbContext(DbContextOptions options) : base(options)
    {
        ChangeTracker.Tracked += OnEntityTracked;
        ChangeTracker.StateChanged += OnEntityStateChanged;
    }
    
    private void OnEntityTracked(object sender, EntityTrackedEventArgs e)
    {
        Console.WriteLine($"Entity tracked: {e.Entry.Entity.GetType().Name}");
    }
    
    private void OnEntityStateChanged(object sender, EntityStateChangedEventArgs e)
    {
        Console.WriteLine($"State changed from {e.OldState} to {e.NewState}");
    }
}

Performance Optimization

// Disable tracking for read-only scenarios
services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(connectionString)
           .UseQueryTrackingBehavior(QueryTrackingBehavior.NoTracking));

// Disable change detection
context.ChangeTracker.AutoDetectChangesEnabled = false;
try
{
    // Bulk operations
}
finally
{
    context.ChangeTracker.AutoDetectChangesEnabled = true;
}

Summary

Change tracking in EF Core monitors entity states (Detached, Unchanged, Added, Modified, Deleted) to generate appropriate SQL commands. Use AsNoTracking() for read-only queries to improve performance.