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POLinkedList modified, Thread crashing the program
 

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    <p>My problem is a synchronization problem with a thread and the user simultaneously accessing and modifying a <code>LinkedList</code>.</p> <p>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 <code>LinkedList</code>.</p> <p>I have read some solutions but I am still unable to make them work. I’m using an <code>Enumerator</code> for the thread work in my <code>LinkedList</code>. 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.</p> <p>Here is some code of my project. This one is for <strong>adding a message</strong>:</p> <pre><code>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(); //} } </code></pre> <p><strong>Removing a message</strong>:</p> <pre><code>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(); //} } </code></pre> <p>And there is the problem, in <strong>my thread</strong>:</p> <pre><code>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 &lt; 0) { this.Invoke(new MethodInvoker(() =&gt; { this.Controls.Remove(lb); })); if (labels.Count &gt; 1) this.Invoke(new MethodInvoker(() =&gt; { lb.Location = new Point(lastLb.Right, 0); })); else this.Invoke(new MethodInvoker(() =&gt; { lb.Location = new Point(2000, 0); })); lastLb = lb; this.Invoke(new MethodInvoker(() =&gt; { this.Controls.Add(lb); })); this.Invoke(new MethodInvoker(() =&gt; { this.Refresh(); })); } if (leftLb != null) if (leftLb.Location.X + leftLb.Width - lb.Location.X &lt; -20) this.Invoke(new MethodInvoker(() =&gt; { lb.Location = new Point(leftLb.Right, 0); })); else this.Invoke(new MethodInvoker(() =&gt; { lb.Location = new Point(lb.Location.X - 3, lb.Location.Y); })); leftLb = lb; } System.Threading.Thread.Sleep(30); // } } } </code></pre> <p>As you can see I’m using an <code>GetEnumerator</code> of my labels, what in Java should be the <code>Iterator</code>. With this I shouldn’t be able to iterate the list without problem when the user add or remove messages?</p> <p>Is there a way to synchronize the accesses to the list?</p> <p><strong>EDIT</strong>: I have tried the <code>ConcurrentBag</code> and <code>ConcurrentDictionary</code> but without any improvement to the project as you can see in the comments…</p> <p>Please before you post an answer read the comments bellow to make sure that you know what is going on.</p> <p><strong>EDIT</strong>: Tried to add a mutex to my code for <code>addMsg</code> and <code>removeMsg</code> but still crashing. If I use the mutex in the thread it will be slowed down.</p>
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    1. COWhy not try a ConcurrentQueue, a ConcurrentStack or a ConcurrentBag?
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    2. CODon't do that. It defeats the purpose of using threads.
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    3. CO*that is because I'm adding messages or removing while a thread is accessing the LinkedList*. **No** it is because you add/remove to the list while you are *enumerating* it.
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