Why do these two List always contain the same number of items?

Question

I have a very simple program that for some reason has me stumped. I put it down, came back at it again this morning and I'm still stumped. First off, I'm aware this is not an ideal solution. I have two forms: Main and Log. The Main form has a button that adds to List _debugLog when clicked. When btnDebug is clicked, it opens the Log form, passing _debugLog to it. Everything is fine, the timer is setup and runs, everything is normal. The event log.UpdateLog() is triggered every 2.5 seconds to update the Log form with the updated log. However, mainFormLog.Count and _log.Count are always the same and they BOTH increase when btnAdd is clicked on the main form. How does _log have the new _debugLog (mainFormLog) from the tick event?

namespace Tool
{
    public partial class Main : Form
    {
        private List<string> _debugLog = new List<string>();

        public Main()
        {
            InitializeComponent();
        }

        private void btnAdd_Click(object sender, EventArgs e)
        {
            _debugLog.Add("message!");
        }

        private void btnDebug_Click(object sender, EventArgs e)
        {
            Log log = new Log(_debugLog);
            log.Show();

            Timer dt = new Timer();
            dt.Interval = 2500;
            dt.Enabled = true;

            dt.Tick += delegate {
                log.UpdateLog(_debugLog);
            };

        }
    }

    public partial class Log : Form
    {
        private List<string> _log;

        public Log(List<string> log)
        {
            InitializeComponent();
            _log = log;
        }

        public void UpdateLog(List<string> mainFormLog)
        {
            if (mainFormLog.Count > _log.Count)
            {
                MessageBox.Show("Log has been updated!");
            }
            else
            {
                MessageBox.Show("Nothing new!" + mainFormLog.Count.ToString() + " / " + _log.Count.ToString());
            }
        }
    }
}

you pass _debugLog in the constructor of the Log, that is the reason — Dmitri Tsoy, Nov 23 '16 at 15:51
Can you explain that a little more? I pass it in the constructor, but never again. How does _log keep updating? — Scott, Nov 23 '16 at 15:52
What's up with the down votes? It's a totally valid question. — Good Night Nerd Pride, Nov 23 '16 at 15:53
To better understand what you did wrong, you should read about the difference between values types and reference types in C#: http://www.codeproject.com/Articles/76153/Six-important-NET-concepts-Stack-heap-value-types — Good Night Nerd Pride, Nov 23 '16 at 15:55
Thank you, I will have a read. Tim Schmelter cleared it up for me, I see where the issue is now. — Scott, Nov 23 '16 at 15:57

score 3 · Accepted Answer · edited May 23 '17 at 11:47

Well, you're passing the reference to the list from Main to Log, so it's actually the same list.

If you want a separate list that gets initialized with the list from Main you can use:

public Log(List<string> log)
{
    InitializeComponent();
    _log = new List<string>(log);
}

Maybe this helps to understand the difference between variables and references:

For a value type, the value is the information itself. For a reference type, the value is a reference which may be null or may be a way of navigating to an object containing the information.

For example, think of a variable as like a piece of paper. It could have the value "5" or "false" written on it, but it couldn't have my house... it would have to have directions to my house. Those directions are the equivalent of a reference. In particular, two people could have different pieces of paper containing the same directions to my house - and if one person followed those directions and painted my house red, then the second person would see that change too. If they both just had separate pictures of my house on the paper, then one person colouring their paper wouldn't change the other person's paper at all.

I see now, that makes total sense. – Scott Nov 23 '16 at 15:55 — Scott, Nov 23 '16 at 15:55

score 0 · Answer 2 · answered Nov 23 '16 at 15:51

All your variables _debugLog, mainFormLog, and _log are pointing to the same list in memory. You've only created one list, and when you assign a new variable to that list, it's just a pointer to some location in memory, it doesn't automatically create a new copy of the list.

Why do these two List always contain the same number of items?

2 Answers2