How to configure a one-to-one relationship in EF Core?

Example using Fluent API:

modelBuilder.Entity<User>()

.HasOne(u => u.Profile)

.WithOne(p => p.User)

.HasForeignKey<Profile>(p => p.UserId);

Or with data annotations:

public class Profile
{

[Key, ForeignKey("User")]

public int UserId { get; set; }
public User User { get; set; }
}

• Requires a unique foreign key.
• EF assumes optional by default unless marked as required.