/// <summary>
 /// Creates the new (attribute could not exists before) attribute value 
 /// (<see cref="IsModified"/> of attribute is false).
 /// </summary>
 /// <param name="propertyName">Name of the single property.</param>
 /// <param name="value">The value.</param>
 public void CreatePropertyWithValue(string propertyName, long? value)
 {
     TikPropertyItem item = new TikPropertyItem(value);
     items.Add(propertyName, item);
 }
        private TikPropertyItem GetOrCreateItem(string propertyName)
        {
            TikPropertyItem result;
            if (!items.TryGetValue(propertyName, out result))
            {
                result = new TikPropertyItem();
                items.Add(propertyName, result);
            }

            return result;
        }