Why does my optional one-to-one relationship with Entity Framework Core not work?

I have an existing sql database with 2 tables that have an optional one-to-one relationship with proper records.

I configured the given relationship with Entity Framework Core for my asp.net core web application as given below:

// Principal (parent)
public class Person
{
    [Key]
    public int Id { get; set; }
    public string LastName { get; set; }
    public virtual Address Address { get; set; }
}

// Dependent (child)
public class Address
{
    public int? AddressRef { get; set; }
    public string Street { get; set; }
    public virtual Person Person { get; set; }
}

modelBuilder.Entity<Person>(entity =>
{
    entity.ToTable("Person");
    entity.HasKey(k => new { k.Id });
    entity.Property(p => p.Id)
            .HasColumnName("PersonRef");
});

modelBuilder.Entity<Address>(entity =>
{
    entity.ToTable("Address");
    entity.HasKey(x => x.AddressRef);
    entity.Property(e => e.AddressRef).ValueGeneratedNever();

    entity.HasOne(a => a.Person)
            .WithOne(b => b.Address)
            .HasForeignKey<Address>(c => c.AddressRef)
            .IsRequired(false)                  
            .OnDelete(DeleteBehavior.ClientSetNull).HasConstraintName("FK_dbo.Address_dbo.Person_AddressRef");
})

When I try to add a new Person via DbContext

var newPerson = new Person()
{
    LastName = model.LastName,
    // ...
};
DbContext.Add(newPerson);
DbContext.SaveChanges()   // Error is thrown

I get the ErrorMessage:

> The value of 'Person.Id' is unknown when attempting to save changes. This is because the property is also part of a foreign key for which the principal entity in the relationship is not known.'

I have tried several variations for configuring the one-to-one relationship but all give the same error.

Any help would gratefully be appreciated.

Your principal entity's Primary Key should be a annotated as AutoGeneratedValue.

 modelBuilder.Entity<Person>(entity =>
 {
     entity.ToTable("Person");
     entity.HasKey(k => k.Id); // .ValueGeneratedOnAdd(); problem was here
     entity.Property(k => k.Id).ValueGeneratedOnAdd(); // <-- Here is the change
     entity.Property(k => k.Id)
           .HasColumnName("PersonRef");
 });

Note: Primiary key should have a value for Primary Key in Principal Entity because it will be used as primary key in the Dependent Entity

And as @FFad3 tried to tell, in your scenario Address is Optional not Person as I mentioned above the primary key of the Address should be Mapped from an existing record on Person table. You can't have a record of Address without having a corresponding person.

You may change it to one-to-many relation to make it possible. in this way in Person table you will have a Foreign Key for related Address (nullable foreign key) and separated Primary key on Address entity.

If you ask me I will explain you how to do this.

I think that you should change

public virtual Address Address { get; set; } => public virtual Address? Address { get; set; }

also check migration script if Person.Id is set to SqlServer:Identity

Id = table.Column<int>(type: "int", nullable: false)
.Annotation("SqlServer:Identity", "1, 1")