class Node
{
    public required Node Next { get; set; }
    public required Node Previous { get; set; }
    public string Name {get; init;}
    public Node(string name) => Name = name;
}

public static IEnumerable<string> Iterate(Node start) {
  var current = start;
  do { 
    yield return current.Name; 
    current = current.Next;
  } while(current != start);
}

var node1 = new Node("Hello"); 
var node2 = new Node("World") { Next = node1, Previous = node1 }; 
foreach(var n in Iterate(node1))
  Console.WriteLine(n);

Compile time error: Required member 'Program.Node.Next' must be set in the object initializer or attribute constructor.
Compile time error: Required member 'Program.Node.Previous' must be set in the object initializer or attribute constructor.

var node1 = new Node("Hello") { Next = null!, Previous = null! }; 
var node2 = new Node("World") { Next = node1, Previous = node1) }; 
node1.Previous = node2;
foreach(var n in Iterate(node1)
  Console.WriteLine(n);

S>#продвинутый nullable enable
S>        var obj1 = new SomeType1();
S>       var obj2 = new SomeType2();
S>obj1.Prop = obj2;
S>obj2.Prop = obj1;
S>// Local functions are allowed
S>void Connect() => obj1.Child = obj2;
S>Connect();
S>#продвинутый nullable disable
S>