Extension.cs

    public static class ExtensionMethods
    {
        public static int RemoveAll<T>(this ObservableCollection<T> coll, Func<T, bool> condition)
        {
            var itemsToRemove = coll.Where(condition).ToList();

            foreach (var itemToRemove in itemsToRemove)
            {
                coll.Remove(itemToRemove);
            }

            return itemsToRemove.Count;
        }
    }