LinkedList modified, Thread crashing the program

Question

My problem is a synchronization problem with a thread and the user simultaneously accessing and modifying a LinkedList.

I’m making a program in C# that will display some messages in a panel. I’m getting an error called “The collection was modified after the enumerator was instantiated.”, that is because I’m adding or removing messages while a thread is accessing the LinkedList.

I have read some solutions but I am still unable to make them work. I’m using an Enumerator for the thread work in my LinkedList. I tried to make some locks in my code so the thread would not iterate the list while I remove or add an element. I also tried to lock the thread for the operations on my list. But all my attempts failed.

Here is some code of my project. This one is for adding a message:

public void addMsg(MsgForDisplay msg) {
    Label lbl = new Label();
    lbl.Text = (msg.getMsgText() + " -");
    lbl.ForeColor = color;
    lbl.Font = textFont;
    lbl.BackColor = backg;
    lbl.Visible = true;
    lbl.AutoSize = true;
    lbl.Location = new Point(width(), 0);
    //lock(labels) { tried to lock here but failed
        labels.AddLast(lbl);
        lastLb = lbl;
        this.Controls.Add(lbl);
        this.Refresh();
    //}
}

Removing a message:

public void removeMsg(string msg) {
    string remove = msg + " -";
    Label lbRemove = null;
    //lock(labels) { also tried to lock here
        var it = labels.GetEnumerator();
        while(it.MoveNext()) {
            Label label = it.Current;
            if (label.Text.Equals(remove)) {
                lbRemove = label;
            }
        }
        labels.Remove(lbRemove);
        this.Controls.Remove(lbRemove);
        this.Refresh();
    //}
}

And there is the problem, in my thread:

public void run() {
    while (true) {
        // lock (labels) { also tried to lock here
            var it = labels.GetEnumerator();
            while (it.MoveNext()) { // the crash occurs here
                Label lb = it.Current;
                if (lb.Location.X + lb.Width < 0) {
                    this.Invoke(new MethodInvoker(() => { this.Controls.Remove(lb); }));
                    if (labels.Count > 1)
                        this.Invoke(new MethodInvoker(() => { lb.Location = new Point(lastLb.Right, 0); }));
                    else
                        this.Invoke(new MethodInvoker(() => { lb.Location = new Point(2000, 0); }));
                    lastLb = lb;
                    this.Invoke(new MethodInvoker(() => { this.Controls.Add(lb); }));
                    this.Invoke(new MethodInvoker(() => { this.Refresh(); }));
                }
                if (leftLb != null)
                    if (leftLb.Location.X + leftLb.Width - lb.Location.X < -20)
                        this.Invoke(new MethodInvoker(() => { lb.Location = new Point(leftLb.Right, 0); }));
                    else
                        this.Invoke(new MethodInvoker(() => { lb.Location = new Point(lb.Location.X - 3, lb.Location.Y); }));
                    leftLb = lb;
            }
            System.Threading.Thread.Sleep(30);
        // }
    }
}

As you can see I’m using an GetEnumerator of my labels, what in Java should be the Iterator. With this I shouldn’t be able to iterate the list without problem when the user add or remove messages?

Is there a way to synchronize the accesses to the list?

EDIT: I have tried the ConcurrentBag and ConcurrentDictionary but without any improvement to the project as you can see in the comments…

Please before you post an answer read the comments bellow to make sure that you know what is going on.

EDIT: Tried to add a mutex to my code for addMsg and removeMsg but still crashing. If I use the mutex in the thread it will be slowed down.

Answer 1

I created a Timer in step of the thread and that solved the crashing problem. Here is the code if you want to see it.

private System.Windows.Forms.Timer myTimer = new System.Windows.Forms.Timer();

private void startThread() {
    myTimer.Tick += new EventHandler(timerEvent);
    myTimer.Interval = 30;
    myTimer.Start();
}

private void timerEvent(object sender, EventArgs e) {
    var it = labels.GetEnumerator();

    while (it.MoveNext()) {
        Label lb = it.Current;
        // Label lb = labels.ElementAt(b);
        if (lb.Location.X + lb.Width < 0) {
            this.Controls.Remove(lb);
            if (labels.Count > 1)
                lb.Location = new Point(lastLb.Right, 0);
            else
                lb.Location = new Point(2000, 0);
            lastLb = lb;
            this.Controls.Add(lb);

            this.Refresh();
        }

        if (leftLb != null)
            if (leftLb.Location.X + leftLb.Width - lb.Location.X < -20)
                lb.Location = new Point(leftLb.Right, 0);
            else
                lb.Location = new Point(lb.Location.X - 3, lb.Location.Y);
        leftLb = lb;
    }
}