Why does enumerating through a collection throw an

Question

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.

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.

Code is below:

   class Program
   {
    private static List<string> _collection = new List<string>();
    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 <= 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 < _collection.Count; i++)
        //{
        //    Console.WriteLine("Reading " + _collection[i]);
        //    System.Threading.Thread.Sleep(150);
        //}

        // This will throw an exception
        List<string>.Enumerator enumerator = _collection.GetEnumerator();
        while (enumerator.MoveNext())
        {
            string value = enumerator.Current;
            System.Threading.Thread.Sleep(150);
            Console.WriteLine("Reading " + value);
        }
    }
}

Answer 1

The code is flawed, in that you are sleeping for 5 seconds but not all of the items have been added to the list. This means you start to display the items on one thread before the first thread has finished adding items to the list, causing the underlying collection to chnage and invalidating the enumerator.

Removing the Thread.Sleep from the Add code highlights this:

public static void AddItems(object state_)
{     
   for (int i = 1; i <= 50; i++)      
   {        
       _collection.Add(i.ToString());      
       Console.WriteLine("Adding " + i);  
   }   
} 

Rather than sleeping you should use a synchronisation mechanism that waits for the first thread to complete it's work of adding items.