private async Task NotifyRemoved(Entry change)
 {
     foreach (var subscriber in subscribers)
     {
         await subscriber.Value.NotifyRemoved(change).ConfigureAwait(false);
     }
 }
 public Task NotifyRemoved(Entry removedEntry)
 {
     if (type == removedEntry.Type)
     {
         return onRemoved(removedEntry);
     }
     return Task.FromResult(0);
 }
 public Task NotifyChanged(Entry changedEntry)
 {
     if (type == changedEntry.Type)
     {
         return onAddedOrUpdated(changedEntry);
     }
     return Task.FromResult(0);
 }