two way mapping

Is any way, how to create two classes, which will be referenced both way and will use only one FK? This interest me in One-to-One as like as One-to-Many cases.

f.e.:

Class First: Entity
{
Second second;
}

Class Second: Entity
{
First first;
}

String TwoWayReference()
{
First Fir = new First();
Second Sec = new Second();

Fir.second = Se开发者_StackOverflow社区c; // I need it is equivalent to: Sec.first = Fir;

if (Sec.first == Fir)
    return "Is any way how to do this code works and code return this string?";
else
    return "Or it is impossible?"
}

simplest would be

class First : Entity
{
    private Second second;
    public virtual Second Second
    {
        get { return this.second; }
        set {
            if (value != null)
            {
                value.First = this;
                this.second = value;
            }
        }
    }
}

class Second : Entity
{
    private First first;
    public virtual First First
    {
        get { return this.first; }
        set {
            if (value != null && value.Second != this)
            {
                value.Second = this;
                this.first = value;
            }
        }
    }
}