Umbraco v13+ 404 Error IContentLastChanceFinder - Supporting multiple sites at root

Error404Page.cs

namespace MYSITE.Web;


using Umbraco.Cms.Core.Composing;
using Umbraco.Cms.Core.DependencyInjection;
using Umbraco.Cms.Core.Models.PublishedContent;
using Umbraco.Cms.Core.Routing;
using Umbraco.Cms.Core.Web;
using Umbraco.Extensions;

using MYSITE.Web.Models; //For the 'ErrorPage' Content Type Model

public class Error404Page : IContentLastChanceFinder
{
	private readonly IUmbracoContextAccessor _contextAccessor;

	public Error404Page(IUmbracoContextAccessor contextAccessor)
	{
		_contextAccessor = contextAccessor;
	}

	public Task<bool> TryFindContent(IPublishedRequestBuilder request)
	{
		// In the rare case that an umbracoContext cannot be build from the request, we will not be able to find the page
		if (_contextAccessor.TryGetUmbracoContext(out var umbracoContext) == false)
		{
			return Task.FromResult(false);
		}

		// Find the first notFound page at root level through the published content cache by its documentTypeAlias
		// You can make this search as complex as you want, you can return different pages based on anything in the original request

		IPublishedContent? notFoundPage = null;
		//var rootContentNodes = umbracoContext.Content?.GetAtRoot();
		var currentCulture = request.Culture;

		var realPage = GetClosestContent(umbracoContext, request.Uri); //Include currentCulture param if multi-lingual
		var site = realPage.AncestorOrSelf(1); //Website is at Level 1 (usually) 
		var errorPages = site.Descendants<ErrorPage>().ToList(); // you can also use .Where(c => c.ContentType.Alias == "myErrorPageTypeAlias")
		if (errorPages.Count() == 1)
		{
			notFoundPage = errorPages.First();
		}
		else if (errorPages.Count() > 1)
		{
			var match404 = errorPages.First(n => n.Name == "404");
			if (match404 != null)
			{
				notFoundPage = match404;
			}
			else
			{
				notFoundPage = errorPages.First();
			}
		}
		
		if (notFoundPage != null)
		{
			// set the content on the request and mark our search as successful
			request.SetPublishedContent(notFoundPage);
			return Task.FromResult(true);
		}

		//Couldn't find anything
		return Task.FromResult(false);
	}

	private IPublishedContent GetClosestContent(IUmbracoContext UmbracoContext, Uri RequestUri,
		string? CurrentCulture = null)
	{

		//Loop backwards testing each possible ancestral path until a real content node is found
		for (int i = RequestUri.Segments.Length - 1; i <= 0; i--)
		{
			var path = string.Join("/", RequestUri.Segments.Take(i));
			var node = UmbracoContext.Content?.GetByRoute(path, false, CurrentCulture);
			if (node != null)
			{

				return node;
			}
		}

		//if we get here, just return the first root node
		return UmbracoContext.Content?.GetAtRoot().FirstOrDefault();
	}
}

// ContentFinders need to be registered into the DI container through a composer
public class Error404PageComposer : IComposer
{
	public void Compose(IUmbracoBuilder builder)
	{
		builder.SetContentLastChanceFinder<Error404Page>();
	}
}