using System;

public class Program

{

    public static void Main()

    {

        Tree tree = new Tree();

        tree.Name = "dub";

        Console.WriteLine("Main: " + (tree?.Name ?? "null"));

        DeleteTree(tree);

        Console.WriteLine("Main: " + (tree?.Name ?? "null"));

    }

    private static void DeleteTree(Tree tree)

    {

        tree = null;

        Console.WriteLine("DeleteTree: " + (tree?.Name ?? "null"));

    }

    public class Tree

    {

        public string Name { get; set; }

    }

}