How to make one to zero/one in Entity Framework?

Question

ASP.NET

There is a main table accounts, which includes various services.
Services have a ratio to this account as 1 to 0/1,
i.е. The account must be, and the service may or may not be.

public class ApplicationUser : IdentityUser<long, VRSIdentityUserLogin, VRSIdentityUserRole, VRSIdentityUserClaim>
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public override long Id { get; set; }
public long Balance { get; set; }
public virtual ServiceEntity1 ServiceEntity{ get; set; }
public virtual ServiceEntity2 ServiceEntity{ get; set; }
}

Services must have data bound to them, some one-to-one, others one-to-many.

public class ServiceEntity1 
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public long Id { get; set; }
public ApplicationUser  User { get; set; }
public virtual  Metrica ServiceMetrica{ get; set; }
public virtual  List<ModuleService> ServiceModules{ get; set; }
}

public class Metrica
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public long id { get; set; }

public virtual long ServiceEntityId { get; set; }
public virtual ServiceEntity1 ServiceEntity { get; set; }
}

It is also necessary that the ApplicationUser.Id be generated by the database and used in ServiceEntity1.Id, which in turn is the key for Metrica.ServiceEntityId

Answer 1

public class ServiceEntity1 
{
[Key, ForeignKey("ApplicationUser")]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public long Id { get; set; }
public ApplicationUser  User { get; set; }

Answer 2

Use the notation "type?" for the secondary key. For example: Guid? or int?. EF will automatically figure out what you want 0..1.