Reference to List question .

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So apparently because of my lack of understanding of reference types.
when I try to create a random list the original list is a duplicate of the
random one. I'm sure I will feel stupid for the answer but here is the code -

class TwoLists
{
    List<Player> _players;
    public List<Player> Players
    {
        get { return _players; }
    }
    //constructor
    public TwoLists()
    {
        _players = new List<Player>();
        CreateLists();
    }
    private void CreateLists()
    {
        for (int i = 0; i < 10; i++)
        {
            Player p = new Player("Player " + i.ToString());
            _players.Add(p);
        }
        CreateRandomList();
    }
    private void CreateRandomList()
    {
        List<Player> tempList = new List<Player>();
        tempList = _players; //  <<<<<<<<<
        Player tempPlayer = new Player();
        Random r = new Random();
        int pCount = _players.Count - 1;
        int rNum = 0;
        for (int i = 0; i <= pCount; i++)
        {
            for (int j = 0; j < 100; j++)
            {
                rNum = r.Next(0, pCount);
                tempPlayer = tempList[i];
                tempList[i] = tempList[rNum];
                tempList[rNum] = tempPlayer;
            }
        }
                // I DID NOT TELL _players TO = templist .
    }
}

So Now when I access Players it is randomized even though I have not randomized it.
so tempList is = to _players. then I work with templist and _players
duplicates it. so My question is how do I leave _players intact.
and what is going on here. obviously I should understand this by now and
so mabey Your answer will make me see what I surely should have aready understood.
and I appreciate your time. also maybey good for other beginners.

Posted 17-Jul-11 4:15am

o1010wanabe

Updated 17-Jul-11 4:20am

v2

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Espen Harlinn · Accepted Answer · 2011-07-17T04:24:00

Solution 1

tempList = _players anything you do to tempList you do to _players, they are both referencing the same object.

You need to create a new list, then make a copy of each element.

Best regards
Espen Harlinn

Posted 17-Jul-11 4:24am

Espen Harlinn

Comments

o1010wanabe 17-Jul-11 10:41am

Thanks Espen .

Espen Harlinn 17-Jul-11 11:10am

Thank you, o1010wanabe

thatraja 17-Jul-11 11:02am

5!

Espen Harlinn 17-Jul-11 11:11am

Than you, thatraja :)

Sergey Alexandrovich Kryukov 18-Jul-11 4:26am

Correct, a 5. Needs a shallow clone.
--SA

Espen Harlinn 18-Jul-11 6:01am

Thank you, Sergey!

Uday P.Singh 18-Jul-11 5:09am

correct my 5 too!!

Espen Harlinn 18-Jul-11 6:01am

Thank you, Uday!

OriginalGriff · Accepted Answer · 2011-07-17T04:30:00

Well, actually, you did.
This is not going to be too easy, without pictures and a quick check every now and then to see if your eyes are glazing over, but here goes...

You have a variable "players" which refers to a list of Player objects. Note that "players" is not the list itself, it is just a reference to it. The difference is that your list could be a set of physical cards containing names and addresses, and "players" could be a piece of paper saying "see card index set 'Names and Addresses'". You can have many pieces of paper, all referring to the same set of cards, but they are all different sheets of paper.

When you create a new list variable,

List<player> tempList = new List<player>();

You create a new sheet of paper, but this time it says "see card index set 'Temporary'" - it refer to a different set of cards, currently empty.
You then do this:

tempList = _players; //  <<<<<<<<<

All this does is use a pencil eraser on the "tempList" sheet, running out all the information, and writing in "see card index set 'Names and Addresses'". It copies the Reference, not the content.

So when you randomize the set of card referred to by the "tempList" sheet, you are also randomizing the set of cards refered to by the "players" sheet (because they both refer to the same cards).

If you want to work with a different set of cards, then you have to copy the list to your new location. Try:

List<player> tempList = new List<player>();
tempList.AddRange(_players);
Player tempPlayer = new Player();

This will create a new list all referring to the same cards (because each item in the list is not the card, it is also a reference to the card - ah, look, your eyes are glazing over).

Does that make sense? I hope it does, because this is a very important concept, and if you don't understand it you are going to cause yourself real trouble in the future!

[edit]Fixed < and >, removed spurious closing tags - OriginalGriff[/edit]