fix: check for NullRepresentationInCache in AppCacheExtensions (#19350)

* fix: add appcache null check

* Moved constant into standard location.
Removed now unnecessary comment.

---------

Co-authored-by: Andy Butland <abutland73@gmail.com>

src/Umbraco.Core/Cache/AppCacheExtensions.cs

@@ -41,7 +41,7 @@ public static class AppCacheExtensions
     public static T? GetCacheItem<T>(this IAppCache provider, string cacheKey)
     {
         var result = provider.Get(cacheKey);
-        if (result == null)
+        if (IsRetrievedItemNull(result))
         {
             return default;
         }
@@ -52,7 +52,7 @@ public static class AppCacheExtensions
     public static T? GetCacheItem<T>(this IAppCache provider, string cacheKey, Func<T> getCacheItem)
     {
         var result = provider.Get(cacheKey, () => getCacheItem());
-        if (result == null)
+        if (IsRetrievedItemNull(result))
         {
             return default;
         }
@@ -60,6 +60,8 @@ public static class AppCacheExtensions
         return result.TryConvertTo<T>().Result;
     }
 
+    private static bool IsRetrievedItemNull(object? result) => result is null or (object)Cms.Core.Constants.Cache.NullRepresentationInCache;
+
     public static async Task<T?> GetCacheItemAsync<T>(
         this IAppPolicyCache provider,
         string cacheKey,

src/Umbraco.Core/Constants-Cache.cs

@@ -1,4 +1,4 @@
-namespace Umbraco.Cms.Core;
+namespace Umbraco.Cms.Core;
 public static partial class Constants
 {
     public static class Cache
@@ -9,5 +9,13 @@ public static partial class Constants
 
             public const string Media = "media";
         }
+
+        /// <summary>
+        /// Defines the string used to represent a null value in the cache.
+        /// </summary>
+        /// <remarks>
+        /// Used in conjunction with the option to cache null values on the repository caches, so we
+        /// can distinguish a true null "not found" value and a cached null value.</remarks>
+        public const string NullRepresentationInCache = "*NULL*";
     }
 }

src/Umbraco.Infrastructure/Cache/DefaultRepositoryCachePolicy.cs

@@ -24,8 +24,6 @@ public class DefaultRepositoryCachePolicy<TEntity, TId> : RepositoryCachePolicyB
     private static readonly TEntity[] _emptyEntities = new TEntity[0]; // const
     private readonly RepositoryCachePolicyOptions _options;
 
-    private const string NullRepresentationInCache = "*NULL*";
-
     public DefaultRepositoryCachePolicy(IAppPolicyCache cache, IScopeAccessor scopeAccessor, RepositoryCachePolicyOptions options)
         : base(cache, scopeAccessor) =>
         _options = options ?? throw new ArgumentNullException(nameof(options));
@@ -139,10 +137,8 @@ public class DefaultRepositoryCachePolicy<TEntity, TId> : RepositoryCachePolicyB
             return fromCache;
         }
 
-        // Because TEntity can never be a string, we will never be in a position where the proxy value collides withs a real value.
-        // Therefore this point can only be reached if there is a proxy null value => becomes null when cast to TEntity above OR the item simply does not exist.
         // If we've cached a "null" value, return null.
-        if (_options.CacheNullValues && Cache.GetCacheItem<string>(cacheKey) == NullRepresentationInCache)
+        if (_options.CacheNullValues && Cache.GetCacheItem<string>(cacheKey) == Constants.Cache.NullRepresentationInCache)
         {
             return null;
         }
@@ -273,7 +269,7 @@ public class DefaultRepositoryCachePolicy<TEntity, TId> : RepositoryCachePolicyB
         // a value that does exist but isn't yet cached, or a value that has been explicitly cached with a null value.
         // Both would return null when we retrieve from the cache and we couldn't distinguish between the two.
         // So we cache a special value that represents null, and then we can check for that value when we retrieve from the cache.
-        Cache.Insert(cacheKey, () => NullRepresentationInCache, TimeSpan.FromMinutes(5), true);
+        Cache.Insert(cacheKey, () => Constants.Cache.NullRepresentationInCache, TimeSpan.FromMinutes(5), true);
     }
 
     protected virtual void InsertEntities(TId[]? ids, TEntity[]? entities)