Cannot set an object to null inside a method [duplicate]

Question

In the below code I am trying to set node to null in test method. When I do that the node becomes Null only inside the scope of the method, but 'parent' doesn't become null. My understanding was objects are passed to methods as reference and can be directly manipulated inside the method. I think there is something wrong in my conceptual understanding. Can you explain why assigning the node to null doesn't assign parent to null.

class Program
{
    static void Main(string[] args)
    {
        Node parent = new Node();
        parent.key = 50;
        parent.left = new Node();
        Test.test(parent);
    }
}

class Test
 {
    public static void test(Node node)
    {
       node.key = 1111;
       node = null;
    }
 }

class Node
    {
        public object key { get; set; }
        public Node left = null;
        public Node right = null;
    }

Answer 1

You're not actually passing parent to node by reference here. This means a value of parent is being copied to node in the test function. In this case, that value simply points to a Node object.

node.key = 1111 works as you expect because it uses that value to access the same object which parent is also pointing to. i.e. both node and parent contain values pointing to the same locations in memory. As such, both can observe the modification.

However when you say node = null, you're assigning a new value to the node variable inside the test function. This means you're changing the pointer stored as the value in that particular variable to null, which in no way modifies the value for parent - that's still pointing to the Node object.

Mind my amateur ASCII art, but I think it's kind of like this:

    Test.test(parent);       

         <node object>
           ^       ^ 
           .           .
           .               .
           .                   .
    +------.----+                   .  +-----------+
    |      .    |                       .          |
    |      .    |    (-> copied to)    |    .      |
    |  parent   |                      |    node   |
    +-----------+                      +-----------+
    Program.Main scope                 Test.test scope


    node = null;       

         <node object>
           ^ 
           .
           .
           .
    +------.----+                      +-----------+
    |      .    |                      |           |
    |      .    |                      |           |
    |  parent   |                      | node=null |
    +-----------+                      +-----------+
    Program.Main scope                 Test.test scope

Whereas if you happened to use public static void test(ref Node node) you could think of it more like this:

    Test.test(parent);       

         <node object>
           ^ 
           .
           .
           .
    +------.----+                      +-----------+
    |  parent <============================ node   |
    |           |                      |           |
    |           |                      |           |
    +-----------+                      +-----------+
    Program.Main scope                 Test.test scope


    node = null;       

         <node object>
            Lonely


    +-----------+                      +-----------+
    |  parent <============================ node   |
    |     =     |                      |           |
    |    null   |                      |           |
    +-----------+                      +-----------+
    Program.Main scope                 Test.test scope