Comparing the behaviour between IEnumerable<T> and List<T>

ObservableCollection Overloads

public ObservableCollection(IEnumerable<T> collection)
{
    this._monitor = new SimpleMonitor<T>();
    if (collection == null)
    {
        throw new ArgumentNullException("collection");
    }
    this.CopyFrom(collection);
}

public ObservableCollection(List<T> list) 
   : base((list != null) ? new List<T>(list.Count) : list)
{
    this._monitor = new SimpleMonitor<T>();
    this.CopyFrom(list);
}

// which both use
 
private void CopyFrom(IEnumerable<T> collection)
{
    IList<T> items = base.Items;
    if ((collection != null) && (items != null))
    {
        using (IEnumerator<T> enumerator = collection.GetEnumerator())
        {
            while (enumerator.MoveNext())
            {
                items.Add(enumerator.Current);
            }
        }
    }
}