class A
{
    public int Id { get; set; }
    public System.Collections.Generic.List<B> BItems { get; set; }
}

class B
{
    public int Id { get; set; }
    public A Parent { get; set; }
    public System.Collections.Generic.List<C> CItems { get; set; }
}

class C
{
    public int Id { get; set; }
    public B Parent { get; set; }
    public System.Collections.Generic.List<D> DItems { get; set; }
}

class D
{
    public int Id { get; set; }
    public int Type { get; set; }
    public C Reference { get; set; }
    public C Parent { get; set; }
}

class ADescription
{
    public int Id { get; set; }
    public System.Collections.Generic.List<BDescription> BDescriptionItems { get; set; }
}

class BDescription
{
    public int Id { get; set; }
    public ADescription Parent { get; set; }
    public System.Collections.Generic.List<CDescription> CDescriptionItems { get; set; }
}

class CDescription
{
    public int Id { get; set; }
    public BDescription Parent { get; set; }
    public System.Collections.Generic.List<DDescription> DDescriptionItems { get; set; }
}

class DDescription
{
    public int Id { get; set; }
    public int Type { get; set; }
    public CDescription Reference { get; set; }
    public CDescription Parent { get; set; }
}

List<A> aList = new List<A>();
A a = new A()
{
    Id = 1,
    BItems = new System.Collections.Generic.List<B>()
};

B b = new B()
{
    Id = 11,
    Parent = a,
    CItems = new System.Collections.Generic.List<C>()
};
a.BItems.Add(b);

C c = new C() 
{
    Id = 111,
    Parent = b,
    DItems = new System.Collections.Generic.List<D>()
};
c.DItems.Add(new D() 
{ 
    Id = 1111, 
    Parent = c
});
c.DItems.Add(new D()
{
    Id = 1112,
    Parent = c
});            
b.CItems.Add(c);
            

b = new B()
{
    Id = 12,
    Parent = a,
    CItems = new System.Collections.Generic.List<C>()
};
a.BItems.Add(b);

b = new B()
{
    Id = 13,
    Parent = a,
    CItems = new System.Collections.Generic.List<C>()
};
a.BItems.Add(b);
            
c = new C()
{
    Id = 131,
    Parent = b,
    DItems = new System.Collections.Generic.List<D>()
};
b.CItems.Add(c);
c = new C()
{
    Id = 132,
    Parent = b,
    DItems = new System.Collections.Generic.List<D>()
};
b.CItems.Add(c);

a.BItems[0].CItems[0].DItems[0].Reference = c;
aList.Add(a);