Umbraco V9 Event to Notification Handler Example

DontShout.cs

using Umbraco.Cms.Core.Events;
using Umbraco.Cms.Infrastructure.Services.Notifications;

namespace TestingEvents
{
    public class DontShout : INotificationHandler<ContentPublishingNotification>
    {
        public void Handle(ContentPublishingNotification notification)
        {
            foreach (var node in notification.PublishedEntities)
            {
                if (node.ContentType.Alias == "corporateNewsAnnouncement")
                {
                    var newsArticleTitle = node.GetValue<string>("newsTitle");
                    if (newsArticleTitle.Equals(newsArticleTitle.ToUpper()))
                    {
                        // Stop putting news article titles in upper case, so cancel publish
                        // notification.Cancel = true;

                        // Explain why the publish event is cancelled
                        // notification.Messages.Add(new EventMessage("Corporate style guideline infringement", "Don't put the news article title in upper case, no need to shout!", EventMessageType.Error));

                        // Above two lines - simplified
                        notification.CancelOperation(new EventMessage("Corporate style guideline infringement", "Don't put the news article title in upper case, no need to shout!", EventMessageType.Error));
                    }
                }
            }
        }
    }
}

Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using System;
using TestingEvents;
using Umbraco.Cms.Core.DependencyInjection;
using Umbraco.Cms.Infrastructure.Services.Notifications;
using Umbraco.Extensions;

namespace Umbraco.Cms.Web.UI.NetCore
{
    public class Startup
    {
        private readonly IWebHostEnvironment _env;
        private readonly IConfiguration _config;

        /// <summary>
        /// Initializes a new instance of the <see cref="Startup"/> class.
        /// </summary>
        /// <param name="webHostEnvironment">The Web Host Environment</param>
        /// <param name="config">The Configuration</param>
        /// <remarks>
        /// Only a few services are possible to be injected here https://github.com/dotnet/aspnetcore/issues/9337
        /// </remarks>
        public Startup(IWebHostEnvironment webHostEnvironment, IConfiguration config)
        {
            _env = webHostEnvironment ?? throw new ArgumentNullException(nameof(webHostEnvironment));
            _config = config ?? throw new ArgumentNullException(nameof(config));
        }



        /// <summary>
        /// Configures the services
        /// </summary>
        /// <remarks>
        /// This method gets called by the runtime. Use this method to add services to the container.
        /// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        /// </remarks>
        public void ConfigureServices(IServiceCollection services)
        {
#pragma warning disable IDE0022 // Use expression body for methods
            services.AddUmbraco(_env, _config)
                .AddBackOffice()
                .AddWebsite()
                .AddComposers()
                .AddNotificationHandler<ContentPublishingNotification, DontShout>()  // Added this to register this with Umbraco
                .Build();
#pragma warning restore IDE0022 // Use expression body for methods
        }

        /// <summary>
        /// Configures the application
        /// </summary>
        public void Configure(IApplicationBuilder app)
        {
            app.UseUmbracoBackOffice();
            app.UseUmbracoWebsite();
        }
    }
}