Shadow Properties in EF Core

What are Shadow Properties?

Shadow properties are properties that exist in the EF Core model but not in the entity class. They’re stored in the database but accessed through the Change Tracker API.

Creating Shadow Properties

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>()
        .Property<DateTime>("CreatedAt");
    
    modelBuilder.Entity<Product>()
        .Property<DateTime>("UpdatedAt");
    
    modelBuilder.Entity<Product>()
        .Property<string>("CreatedBy")
        .HasMaxLength(100);
}

Accessing Shadow Properties

var product = new Product { Name = "Laptop", Price = 999.99m };
context.Products.Add(product);

// Set shadow property
context.Entry(product).Property("CreatedAt").CurrentValue = DateTime.UtcNow;
context.Entry(product).Property("CreatedBy").CurrentValue = "admin";

await context.SaveChangesAsync();

// Read shadow property
var createdAt = context.Entry(product).Property("CreatedAt").CurrentValue;

Querying Shadow Properties

var products = context.Products
    .Where(p => EF.Property<DateTime>(p, "CreatedAt") >= DateTime.Today)
    .ToList();

var recentProducts = context.Products
    .OrderByDescending(p => EF.Property<DateTime>(p, "UpdatedAt"))
    .Take(10)
    .ToList();

Automatic Audit Trail

public override int SaveChanges()
{
    foreach (var entry in ChangeTracker.Entries())
    {
        if (entry.State == EntityState.Added)
        {
            entry.Property("CreatedAt").CurrentValue = DateTime.UtcNow;
            entry.Property("CreatedBy").CurrentValue = _currentUser;
        }
        
        if (entry.State == EntityState.Modified)
        {
            entry.Property("UpdatedAt").CurrentValue = DateTime.UtcNow;
            entry.Property("UpdatedBy").CurrentValue = _currentUser;
        }
    }
    
    return base.SaveChanges();
}

Foreign Keys as Shadow Properties

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Post>()
        .HasOne(p => p.Blog)
        .WithMany(b => b.Posts)
        .HasForeignKey("BlogForeignKey"); // Shadow property
}

Summary

Shadow properties exist in the EF Core model but not in entity classes. They’re useful for audit fields, foreign keys, and metadata. Access them using Entry().Property() or EF.Property() in queries.