class Child{
  public Child(Parent parent){Parent=parent;}
  public Parent Parent {get; private set;}
}
class Parent{
  // singleton child
  public Child Child {get;set;}
  //children
  private List<Child> _children=new List<Child>();
  public List<Child> Children { get {return _children;}}
}

var child = new Child(parent);
parent.Children.Add(child);

var child = new Child(parent1);
parent2.Children.Add(child);

class Child{
  public Child(Parent parent){Parent=parent;}
  public Parent Parent {get; private set;}
}
class Parent{
  // singleton child
  private Child _child;
  public Child Child {get{return _child;}
    set{
      if (value.Parent != this) throw new Exception();
      _child=value;}}
  //children
  private List<Child> _children=new List<Child>();
  public ReadOnlyCollection<Child> Children { get {return _children;}}
  public void AddChild(Child child){
    if (child.Parent != this) throw new Exception();
    _children.Add(child);
}

class Child{
  public Parent Parent {get; internal set;}
}
class Parent{
  // singleton child
  private Child _child;
  public Child Child {get{return _child;}
    set{
      value.Parent=this;
      _child = value;
  }}
  //children
  private List<Child> _children=new List<Child>();
  public ReadOnlyCollection<Child> Children { get {return _children;}}
  public void AddChild(Child child){
    child.Parent=this;
    _children.Add(child);
  }
}

class Child{
  internal Child(Parent parent, init-params){Parent=parent;}
  public Parent Parent {get; private set;}
}
class Parent{
  // singleton child
  public Child Child {get; private set;}
  public void CreateChild(init-params) {
      var child = new Child(this, init-params);
      Child = value;
  }
  //children
  private List<Child> _children=new List<Child>();
  public ReadOnlyCollection<Child> Children { get {return _children;}}
  public Child AddChild(init-params){
    var child = new Child(this, init-params);
    _children.Add(child);
    return child;
  }
}

new Child(){prop=value}

var c = parent.AddChild();
c.prop=value;

//singleton
class Child{
  public Child(ParentWithChild parent){
    Parent=parent;
    Parent.Child=this;
  }
  public ParentWithChild Parent {get; private set;}
}
class ParentWithChild{
  public Child Child {get;internal set;}
}

//children
class Child{
  public Child(ParentWithChildren parent){
    Parent=parent;
    Parent._children.Add(this);
  }
  public ParentWithChildren Parent {get; private set;}
}
class ParentWithChildren{
  internal List<Child> _children=new List<Child>();
  public ReadOnlyCollection<Child> Children { get {return _children;}}
}

var parent = new ParentWithChildren();
new Child(parent); //adds child to parent
new Child(parent);
new Child(parent);

var parent = new ParentWithChild();
new Child(parent); // sets parent.Child

class Child{
  public Child(Parent parent){
    Parent=parent;
    Parent._children.Add(this);
  }
  public Parent Parent {get; private set;}
}
class Parent{
  //children
  internal List<Child> _children=new List<Child>();
  public ReadOnlyCollection<Child> Children { get {return _children;}}
}