protected virtual void OnAdding(AddingEventArgs <TKey, TValue> args)
 {
     if (this.Adding != null)
     {
         Adding(this, args);
     }
 }
 private static void SortedDict_Adding(object sender, AddingEventArgs <int, string> e)
 {
     Console.WriteLine($"before adding key: [{e.Key}] value: [{e.Value}]");
 }