Restricting the total number of items in an EPiServer media folder

ValidateFileCountInit.cs

using System.Linq;
using EPiServer.Core;
using EPiServer.Framework;
using EPiServer.Framework.Initialization;
using EPiServer.Framework.Localization;
using EPiServer.ServiceLocation;
using InitializationModule = EPiServer.Web.InitializationModule;

namespace EPiServer.Templates.Alloy.ValidateItemsInFolder
{
    [InitializableModule]
    [ModuleDependency(typeof(InitializationModule))]
    public class ValidateFileCountInit : IInitializableModule
    {
        public void Initialize(InitializationEngine context)
        {
            IContentEvents events = ServiceLocator.Current.GetInstance<IContentEvents>();
            events.CreatingContent += ValidateFileCount;
            events.MovingContent += ValidateFileCount;
        }

        private void ValidateFileCount(object sender, ContentEventArgs e)
        {
            // Check if it's a media item
            if (!(e.Content is MediaData)) return;

            // Should never happen but always worth being defensive
            var repo = ServiceLocator.Current.GetInstance<IContentRepository>();
            if (repo == null) return;

            // Get the folder we are trying to create the media item in
            var targetFolder = e is MoveContentEventArgs ? e.TargetLink : e.Content.ParentLink;
            if (targetFolder == null) return;

            // Check for the total number of media items in the target folder
            var childItems = repo.GetChildren<MediaData>(targetFolder);
            const int maxFileCount = 100; // << Pick a sensible number based on requirements/testing
            if (childItems == null || childItems.Count() <= maxFileCount) 
                return;

            // Get the translated error message
            var lang = ServiceLocator.Current.GetInstance<LocalizationService>();
            string errorMessage;
            if (!lang.TryGetString("/errors/toomanyfiles", out errorMessage))
            {
                errorMessage = "Too many media items in this folder, please create a sub-folder or move media items out of the folder before re-trying";
            }

            // Set up the error
            e.CancelAction = true;
            e.CancelReason = errorMessage;
        }

        public void Preload(string[] parameters) { } // This can be removed if using EPiServer 8+

        public void Uninitialize(InitializationEngine context)
        {
            IContentEvents events = ServiceLocator.Current.GetInstance<IContentEvents>();
            events.CreatingContent -= ValidateFileCount;
        }
    }
}