将字典值设置为 null 的引用类型的正确方法是什么？

Question

Let us suppose we have a class like below:-

        class DoubleLinkedListNode
        {
            public int Value { get; set; }

            public DoubleLinkedListNode(int value)
            {
                Value = value;
            }

            public DoubleLinkedListNode next = null;

            public DoubleLinkedListNode prev = null;
        }

And then we create a dictionary as given below:-

IDictionary<int, DoubleLinkedListNode> dict = new Dictionary<int, DoubleLinkedListNode>();

The key of the dictionary will hold the Value of the DoubleLinkedListNode that it contains like this:-

DoubleLinkedListNode newNode = new DoubleLinkedListNode(value);
dict.Add(newNode.Value, newNode );

Let us also create a function which takes the type of the value of the dictionary as input and we make that value null inside the body of the function as shown below:-

private void RemoveNode(DoubleLinkedListNode nodeToBeRemoved)
{
     if(nodeToBeRemoved != null)
     {
         //Do Something

         nodeToBeRemoved = null;
     }

}

and we call the function like this:-

RemoveNode(dict[someValue]);

Let us create another function in which we make the value null explicitly as shown below:-

    private void RemoveNodeAnother(DoubleLinkedListNode nodeToBeRemoved)
    {
         if(nodeToBeRemoved != null)
         {
             //Do Something
             dict[nodeToBeRemoved.Value] = null;
         }

    }

And then we call the function like this:-

RemoveNodeAnother(dict[someValue]);

What is the difference between the above two functions?

Why I am asking is this I was doing this question on Leetcode. The solution that I wrote is as follows:-

public class FirstUnique 
{
    private class DoubleLinkedListNode
    {
        public int Value { get; set; }

        public DoubleLinkedListNode(int value)
        {
            Value = value;
        }

        public DoubleLinkedListNode next = null;

        public DoubleLinkedListNode prev = null;
    }

    DoubleLinkedListNode dummyHeadNode = new DoubleLinkedListNode(-1);

    DoubleLinkedListNode dummyTailNode = new DoubleLinkedListNode(-1);

    IDictionary<int, DoubleLinkedListNode> dict = new Dictionary<int, DoubleLinkedListNode>();

    public FirstUnique(int[] nums) 
    {
        InitialiseDummyHeadAndTailNodes();

        foreach(int i in nums)
        {
            Add(i);
        }
    }

    public int ShowFirstUnique() 
    {
        return dummyHeadNode.next.Value;
    }

    public void Add(int value)
    {
        if (dict.ContainsKey(value))
        {
            RemoveNode(dict[value]);
        }
        else
        {
            DoubleLinkedListNode newNode = new DoubleLinkedListNode(value);

            AddNode(newNode);

        }
    }

    private void InitialiseDummyHeadAndTailNodes()
    {
        dummyHeadNode.next = dummyTailNode;
        dummyTailNode.prev = dummyHeadNode;
    }

    private void RemoveNode(DoubleLinkedListNode nodeToBeRemoved)
    {
        if(nodeToBeRemoved != null)
        {
            nodeToBeRemoved.prev.next = nodeToBeRemoved.next;
            nodeToBeRemoved.next.prev = nodeToBeRemoved.prev;

            // If I write nodeToBeRemoved = null, the solution won't pass.
            // But if I write dict[nodeToBeRemoved.Value] = null, the solution is accepted.

            dict[nodeToBeRemoved.Value] = null;
        }

    }

    private void AddNode(DoubleLinkedListNode nodeToBeAdded)
    {
        //Update the pointers.
        nodeToBeAdded.prev = dummyTailNode.prev;
        nodeToBeAdded.prev.next = nodeToBeAdded;
        nodeToBeAdded.next = dummyTailNode;
        dummyTailNode.prev = nodeToBeAdded;

        //Add the node to the dictionary.
        dict.Add(nodeToBeAdded.Value, nodeToBeAdded);
    }
}

I have made the comments where the anomaly lies. What could be the reason for this behavior?

Answer 1

There is only one way to "set dictionary value to null" -

 dictionary[key] = null;

indeed, if key is not there you need to add it first... Which leads to the way to set value to null -

 if (dictionary.ContainsKey(key)) 
        dictionary[key] = null;
 else
        dictionary.Add(key, null);

Note that setting value to null has zero impact on what was stored there previously.