Darn, it just occurred to me that I should use the Creating event, doi! That should hopefully solve.

I now have a similar issue with the Creating version, in that it still goes into the document properties, and it no longer shows my notification.

Looking for some hints ;)

    public class Subscription : ApplicationEventHandler
{
    protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        base.ApplicationStarted(umbracoApplication, applicationContext);

        Umbraco.Core.Services.ContentService.Creating += ContentService_Creating;
    }

    private void ContentService_Creating(IContentService sender, NewEventArgs<IContent> e)
    {
        if (e.Alias.ToUpper() == "MAINSIDEBAR")
        {
            e.CancelOperation(new Umbraco.Core.Events.EventMessage("Sidebar", "Only one sidebar allowed", Umbraco.Core.Events.EventMessageType.Error));

            LogHelper.Error(typeof(Subscription), $"Attempt to create additional sidebar", new ApplicationException());
        }
    }
}

The way I've done it in the past has been something like:

private void ContentService_Creating(IContentService sender, NewEventArgs<IContent> e)
{
    if (e.Alias.ToUpper() == "MAINSIDEBAR" && e.CanCancel)
    {
        e.Cancel = true;
        e.Messages.Add(new Umbraco.Core.Events.EventMessage("Sidebar", "Only one sidebar allowed", Umbraco.Core.Events.EventMessageType.Error));
    }
}

Not sure if that works better?

If you want to log, I wouldn't raise an exception - use a warn instead.

Thank you.

This runs, and it executes e.Cancel = true; but nothing happens in the UI/backoffice: the content is saved and appears in the tree.

I also tried using Created, as a notice in VS says that Creating is obsolete, but, again, nothing happens.

I tried using Saving, and it kind-of worked, but generated an additional (404) error and left the backoffice in an indeterminate state (just with a loading icon).

I've reverted to Publishing (with a Warn log) for the moment, but it's not as good as I'd like it :(

        private void ContentService_Publishing(Umbraco.Core.Publishing.IPublishingStrategy sender, Umbraco.Core.Events.PublishEventArgs<Umbraco.Core.Models.IContent> e)
    {
        foreach (var ent in e.PublishedEntities)
        {
            if (ent.ContentType.Alias.ToUpper() == "MAINSIDEBAR")
            {
                e.CancelOperation(new Umbraco.Core.Events.EventMessage("Sidebar", "Only one sidebar allowed", Umbraco.Core.Events.EventMessageType.Error));

                LogHelper.Warn(typeof(Subscription), $"Attempt to create additional sidebar(s) (writer id: {ent.WriterId})");
            }
        }
    }

is working on a reply...