Question

I don’t know why I have an IndexOutOfRangeException when I am clearing a System.Collections.Generic.List<T>. Does this make sense?

List<MyObject> listOfMyObject = new List<MyObject>();
listOfMyObject.Clear(); 

Answer 1

This typically happens if multiple threads are accessing the list simultaneously. If one thread deletes an element while another calls Clear(), this exception can occur.

The “answer” in this case is to synchronize this appropriately, locking around all of your List access.

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Edit:

In order to handle this, the simplest method is to encapsulate your list within a custom class, and expose the methods you need, but lock as needed. You’ll need to add locking to anything that alters the collection.

This would be a simple option:

public class MyClassCollection
{
    // Private object for locking
    private readonly object syncObject = new object(); 

    private readonly List<MyObject> list = new List<MyObject>();
    public this[int index]
    {
        get { return list[index]; }
        set
        {
             lock(syncObject) { 
                 list[index] = value; 
             }
        }
    }

    public void Add(MyObject value)
    {
         lock(syncObject) {
             list.Add(value);
         }
    }

    public void Clear()
    {
         lock(syncObject) {
             list.Clear();
         }
    }
    // Do any other methods you need, such as remove, etc.
    // Also, you can make this class implement IList<MyObject> 
    // or IEnumerable<MyObject>, but make sure to lock each 
    // of the methods appropriately, in particular, any method
    // that can change the collection needs locking
}