EF Core Shadow Properties & Backing Fields

Mind Map Summary

• Topic: EF Core Shadow Properties & Backing Fields
• Core Concepts:
  • Shadow Properties: Properties that are not defined in your .NET entity class but are part of the EF Core model for that entity. They are mapped to columns in the database.
  • Backing Fields: Fields that are used to store the data for a property, rather than the property’s getter and setter. This allows for more control over how the property is accessed.
• Use Cases:
  • Shadow Properties:
    • Foreign Keys: EF Core can create shadow properties for foreign keys by convention.
    • Auditing: Store auditing data like LastUpdated or CreatedBy without exposing it on the entity class.
  • Backing Fields:
    • Read-Only Properties: Create properties that can be written to by EF Core but are read-only in the application code.
    • Validation: Add validation logic to the property’s setter.

Practice Exercise

Configure a LastUpdated shadow property on an entity. Use an interceptor or override SaveChanges to automatically set the value of this shadow property whenever the entity is updated. Then, query the database to confirm the value was saved, even though it’s not on the C# class.

Answer

1. Configure the Shadow Property:

using Microsoft.EntityFrameworkCore;

public class MyDbContext : DbContext
{
    public DbSet<Product> Products { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Product>()
            .Property<DateTime>("LastUpdated");
    }

    public override int SaveChanges()
    {
        var entries = ChangeTracker
            .Entries()
            .Where(e => e.State == EntityState.Modified);

        foreach (var entry in entries)
        {
            entry.Property("LastUpdated").CurrentValue = DateTime.UtcNow;
        }

        return base.SaveChanges();
    }
}

2. Update an Entity:

var product = context.Products.First();
product.Name = "New Name";
context.SaveChanges();

3. Query the Shadow Property:

var lastUpdated = context.Entry(product).Property("LastUpdated").CurrentValue;
Console.WriteLine($"Last Updated: {lastUpdated}");

Explanation:

• We configure the LastUpdated shadow property in the OnModelCreating method.
• We override the SaveChanges method to automatically set the value of the LastUpdated property whenever an entity is modified.
• We can then query the value of the shadow property using the Entry and Property methods of the DbContext.