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POWhy does enumerating through a collection throw an exception but looping through its items does not
 

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  1. POWhy does enumerating through a collection throw an exception but looping through its items does not
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    <p>I was testing out some synchronization constructs and I noticed something that confused me. When I was enumerating through a collection while writing to it at the same time, it threw an exception (this was expected), but when I looped through the collection using a for loop, it did not. Can someone explain this? I thought that a List does not allow a reader and writer to operate at the same time. I would have expected looping through the collection to exhibit the same behavior as using an enumerator.</p> <p><strong>UPDATE: This is a purely academic exercise. I undersand that enumerating a list is bad if it is being written to at the same time. I also understand that I need a synchronization construct. My question again was about why operation one throws an exception as expected but the other does not.</strong></p> <p>Code is below:</p> <pre><code> class Program { private static List&lt;string&gt; _collection = new List&lt;string&gt;(); static void Main(string[] args) { ThreadPool.QueueUserWorkItem(new WaitCallback(AddItems), null); System.Threading.Thread.Sleep(5000); ThreadPool.QueueUserWorkItem(new WaitCallback(DisplayItems), null); Console.ReadLine(); } public static void AddItems(object state_) { for (int i = 1; i &lt;= 50; i++) { _collection.Add(i.ToString()); Console.WriteLine("Adding " + i); System.Threading.Thread.Sleep(150); } } public static void DisplayItems(object state_) { // This will not throw an exception //for (int i = 0; i &lt; _collection.Count; i++) //{ // Console.WriteLine("Reading " + _collection[i]); // System.Threading.Thread.Sleep(150); //} // This will throw an exception List&lt;string&gt;.Enumerator enumerator = _collection.GetEnumerator(); while (enumerator.MoveNext()) { string value = enumerator.Current; System.Threading.Thread.Sleep(150); Console.WriteLine("Reading " + value); } } } </code></pre>
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    1. COWhy does it matter if you use a for or while loop? (Not trying to be a jackass, just wondering)
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    2. COwhat exception do you get?
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    3. COthe difference is not between for vs while. The question is in both cases I am reading a collection while writing to it, so why is the behavior different.
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