U4-11527 Fix concurrency issue

src/Umbraco.Core/ApplicationContext.cs

@@ -1,8 +1,8 @@
-using System;
-using System.Collections.Generic;
+using Semver;
+using System;
+using System.Collections.Concurrent;
 using System.Configuration;
 using System.Threading;
-using Semver;
 using Umbraco.Core.Configuration;
 using Umbraco.Core.Logging;
 using Umbraco.Core.ObjectResolution;
@@ -13,7 +13,7 @@ using Umbraco.Core.Sync;
 
 namespace Umbraco.Core
 {
-	/// <summary>
+    /// <summary>
     /// the Umbraco Application context
     /// </summary>
     /// <remarks>
@@ -270,7 +270,7 @@ namespace Umbraco.Core
                 return _umbracoApplicationUrl;
             }
         }
-
+
         /// <summary>
         /// Resets the url.
         /// </summary>
@@ -280,10 +280,10 @@ namespace Umbraco.Core
         }
 
 	    // ReSharper disable once InconsistentNaming
-	    internal string _umbracoApplicationUrl;
-
-        internal List<string> _umbracoApplicationDomains = new List<string>();
-
+	    internal string _umbracoApplicationUrl;
+
+        internal ConcurrentDictionary<string, string> _umbracoApplicationDomains = new ConcurrentDictionary<string, string>();
+
         internal string _umbracoApplicationDeploymentId;
 
         private Lazy<bool> _configured;
@@ -399,7 +399,7 @@ namespace Umbraco.Core
             }
 
             return version;
-        }
+        }
 
         private void AssertIsNotReady()
         {

src/Umbraco.Core/Sync/ApplicationUrlHelper.cs

@@ -51,10 +51,9 @@ namespace Umbraco.Core.Sync
             if (request != null)
             {
                 var applicationUrl = GetApplicationUrlFromRequest(request);
-                newApplicationUrl = !appContext._umbracoApplicationDomains.Contains(applicationUrl);
+                newApplicationUrl = appContext._umbracoApplicationDomains.TryAdd(applicationUrl, applicationUrl);
                 if (newApplicationUrl)
                 {
-                    appContext._umbracoApplicationDomains.Add(applicationUrl);
                     LogHelper.Info(typeof(ApplicationUrlHelper), string.Format("New ApplicationUrl detected: {0}", applicationUrl));
                 }
             }