using System;

public sealed class Adam : Male
{
    private static Adam instance;

    private Adam() : base()
    {
        Name = "Adam";
    }
    public static Adam GetInstance()
    {
        if (instance == null)
        {
            instance = new Adam();
        }
        return instance;
    }
}
public sealed class Eve : Female
{
    private static Eve instance;

    private Eve() : base()
    {
        Name = "Eve";
    }
    public static Eve GetInstance(Adam adamInstance)
    {
        if (adamInstance == null) throw new ArgumentNullException();

        if (instance == null)
        {
            instance = new Eve();
        }
        return instance;
    }
}
public class Male : Human
{
    public Male() {}
    public Male(string name, Human mother, Human father)
    {
        if (mother is null) throw new ArgumentNullException();
        if (father is null) throw new ArgumentNullException();
        else
        {
            Name = name;
            Mother = mother;
            Father = father;
        }
    }

}
public class Female : Human
{
    public Female() {}
    public Female(string name, Female mother, Male father)
    {
        if(name=="Eve") throw new ArgumentNullException();
        if (mother is null) throw new ArgumentNullException();
        if (father is null) throw new ArgumentNullException();
        else
        {
            Name = name;
            Mother = mother;
            Father = father;
        }
    }
}
public abstract class Human
{
    public string Name;
    public Human Mother { get; set; }
    public Human Father { get; set; }
}
/*
    public void Father_and_mother_are_essential_for_reproduction()
    {
        // There is just 1 way to reproduce
        Assert.AreEqual(1, typeof(Male).GetConstructors(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance)
          .Where(x => x.IsPublic || x.IsAssembly).Count());

        Assert.AreEqual(1, typeof(Female).GetConstructors(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance).
          Where(x => x.IsPublic || x.IsAssembly).Count());

        var adam = Adam.GetInstance();
        var eve = Eve.GetInstance(adam);
        Assert.Throws<ArgumentNullException>(() => new Male("Seth", null, null));
        Assert.Throws<ArgumentNullException>(()=> new Male("Abel", eve, null));
        Assert.Throws<ArgumentNullException>(() => new Male("Seth", null, adam));
        Assert.Throws<ArgumentNullException>(() => new Female("Azura", null, null));
        Assert.Throws<ArgumentNullException>(() => new Female("Awan", eve, null));
        Assert.Throws<ArgumentNullException>(() => new Female("Dina", null, adam));
        Assert.Throws<ArgumentNullException>(() => new Female("Eve", null, null));
        Assert.Throws<ArgumentNullException>(() => new Male("Adam", null, null));
    }
    */