IT:AD:EF/CodeFirst/FluentAPI/Relationships/Examples/1-1 (One-To-One, Required)

   //////1-0..1:
        modelBuilder.Entity<Invoice>()
            .HasRequired(i => i.ShippingAddress)
            .WithMany()
            .WillCascadeOnDelete(false); //1-0: as it can be shared with User, don't bind Invoice and Address, so avoid .WithRequired

Working with:

    public class Address
    {
        public virtual int Id { get; set; }
        public virtual string Street { get; set; }
        //...
    }

    //Requirements to enable the use of change-tracking POCO Proxies:
    //* Class must be public, non-abstract, non-sealed.
    //* Public *virtual* get/set for all persisted properties.
    //* Collection based Navigation properties must implement ICollection<T>
    //Ref: http://bit.ly/KgJPcU
    public class User
    {

        public virtual int Id { get; private set; }
        //Marking *scalar* as virtual, marks it for Proxy change tracking.
        public virtual string FirstName { get; set; }

        //1-1 Relationship:
        public virtual int BillingAddressId { get; set; }
        public virtual Address BillingAddress { get; set; }
        //...
        //Marking a *Navigation Collection* as virtual, marks it for Lazy Loading 
        public virtual ICollection<Invoice> Invoices { get { return _invoices??(_invoices = new Collection<Invoice>()); } private set { _invoices = value; }}
        private ICollection<Invoice> _invoices;
    }