Create circular doubly linked list

Question

I have the next code:

static LinkedList<Entry> items = new LinkedList<Entry>();
public static Entry runner;

Entry is a struct that stores information from a database row.

With the next function I add Entry items to the list:

public void Prepare()
{
    // Makes connection to DB
    while(dbReader.Read())
    {
        Entry entry = new Entry(item1, item n);
        items.AddLast(entry);
    }
    runner = items.First.Value;
}

Then I call a function that correctly sets the values of that runner to the graphic elements on a window.

With two buttons I am able to move forward and backwards on that list while updating all the graphic items correctly, except in the next cases:

When trying to go to the previous node while being at the first node.
When trying to go to the next node while being at the last one.

How would I solve this? I want to make it behave as a circular doubly linked list.
I know that System.Collections.Generic´s LinkedList works as a single/normal/classic doubly linked list.

This is the code of the buttons:

private void previousButton()
{
    runner = items.Find(runner).Previous.Value;
    // Call function to set items<node> to graphical elements
}

nextButton() works exactly the same but uses Next instead of Previous.

I already tried to do runner = items.Find(runner).Previous.Value ?? items.Last.Value; as suggested here, but it says that ?? operator cannot be asigned to types 'Entry' & 'Entry'.

On Prepare() changing runner = items.First.Value; to just runner = items.First(); makes no difference.

Sounds like you need to define your own class that works the way you want and use it instead of `LinkedList`. It's not for us to teach you how to do that or do it for you. Put some thought into the problem, do what you can and come back and ask a question if and when you encounter a specific problem. Keep in mind that you can look at the .NET source code if you want to, so you can see how the `LinkedList` class works and make appropriate changes in your own class. You may even be able to inherit `LinkedList`, although I wouldn't count on it. — user18387401, Jul 31 '22 at 04:00
@user18387401 I know how to make my own class that behaves as expected, I just want to know how to do it while using Microsoft's libraries. — jotaro-gogogo, Jul 31 '22 at 04:03
`runner = items.Find(runner).Previous?.Value ?? items.Last.Value;` — Charlieface, Jul 31 '22 at 04:12

score 1 · Answer 1 · answered Jul 31 '22 at 04:08

Make Entry into a class instead of a struct and you will be able to use the null coalesce operator on it as you expect. Since you are storing references to Entry instances in a LinkedList, you'll likely get better performance from a class than a struct anyways (unless you have a strong need for struct semantics).

score 0 · Accepted Answer · answered Jul 31 '22 at 05:15

I'm not sure why you don't use the exact code provided in the answer at Creating a circularly linked list in C#?

Be that as it may, you can fix your immediate problem by adding a ? null-conditional operator. This is because Previous might be null, and that would be the basis of coalescing to null using ??.

runner = items.Find(runner).Previous?.Value ?? items.Last.Value;

Create circular doubly linked list

2 Answers2