More secure use of linq inside OutputCacheManager.RemoveItems.

The use of linq to generate keys is sometimes producing weird behaviour, like producing NullReferenceExceptions.
We have only this issue on heavy loaded production servers, this will need more investigation.

Author: moonpyk

DevTrends.MvcDonutCaching/KeyBuilder.cs

@@ -32,7 +32,7 @@ public string BuildKey(string controllerName, string actionName)
 
         public string BuildKey(string controllerName, string actionName, RouteValueDictionary routeValues)
         {
-            var builder = new StringBuilder(_cacheKeyPrefix);
+            var builder = new StringBuilder(CacheKeyPrefix);
 
             if (controllerName != null)
             {

DevTrends.MvcDonutCaching/OutputCacheManager.cs

@@ -12,7 +12,8 @@ public class OutputCacheManager : IExtendedOutputCacheManager
         private readonly OutputCacheProvider _outputCacheProvider;
         private readonly IKeyBuilder _keyBuilder;
 
-        public OutputCacheManager() : this(OutputCache.Instance, new KeyBuilder())
+        public OutputCacheManager()
+            : this(OutputCache.Instance, new KeyBuilder())
         {
         }
 
@@ -110,14 +111,27 @@ public void RemoveItems([AspMvcController] string controllerName, [AspMvcAction]
 
             var key = _keyBuilder.BuildKey(controllerName, actionName);
 
-            var keysToDelete = enumerableCache.Where(x => x.Key.StartsWith(key)).Select(x => x.Key);
+            if (string.IsNullOrEmpty(key))
+            {
+                return;
+            }
+
+            var keysToDelete = enumerableCache
+                .Where(_ => !string.IsNullOrEmpty(_.Key) && _.Key.StartsWith(key))
+                .Select(_ => _.Key);
 
             if (routeValues != null)
             {
                 foreach (var routeValue in routeValues)
                 {
-                    var value = routeValue;
-                    keysToDelete = keysToDelete.Where(x => x.Contains(_keyBuilder.BuildKeyFragment(value)));
+                    var keyFrag = _keyBuilder.BuildKeyFragment(routeValue);
+
+                    if (string.IsNullOrEmpty(keyFrag))
+                    {
+                        continue;
+                    }
+
+                    keysToDelete = keysToDelete.Where(_ => !string.IsNullOrEmpty(_) && _.Contains(keyFrag));
                 }
             }