Creates IDisposeOnRequestEnd and ensures UmbracoContext and UmbracoDatabase implement it, then we only dispose of these types of objects at the end of the request if they are part of the httpcontext items (U4-2738). Fixes: U4-2988 UmbracoObjectTypesExtensions is not thread safe

src/Umbraco.Core/IDisposeOnRequestEnd.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Umbraco.Core
+{
+    /// <summary>
+    /// Any class implementing this interface that is added to the httpcontext.items keys or values will be disposed of at the end of the request.
+    /// </summary>
+    public interface IDisposeOnRequestEnd : IDisposable
+    {
+    }
+}

src/Umbraco.Core/Models/UmbracoObjectTypesExtensions.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using Umbraco.Core.CodeAnnotations;
 
@@ -9,7 +10,8 @@ namespace Umbraco.Core.Models
     /// </summary>
     public static class UmbracoObjectTypesExtensions
     {
-        private static readonly Dictionary<UmbracoObjectTypes, Guid> UmbracoObjectTypeCache = new Dictionary<UmbracoObjectTypes,Guid>();
+        //MUST be concurrent to avoid thread collisions!
+        private static readonly ConcurrentDictionary<UmbracoObjectTypes, Guid> UmbracoObjectTypeCache = new ConcurrentDictionary<UmbracoObjectTypes, Guid>();
 
         /// <summary>
         /// Get an UmbracoObjectTypes value from it's name
@@ -48,24 +50,22 @@ namespace Umbraco.Core.Models
         /// <returns>a GUID value of the UmbracoObjectTypes</returns>
         public static Guid GetGuid(this UmbracoObjectTypes umbracoObjectType)
         {
-            if (UmbracoObjectTypeCache.ContainsKey(umbracoObjectType))
-                return UmbracoObjectTypeCache[umbracoObjectType];
+            return UmbracoObjectTypeCache.GetOrAdd(umbracoObjectType, types =>
+                {
+                    var type = typeof(UmbracoObjectTypes);
+                    var memInfo = type.GetMember(umbracoObjectType.ToString());
+                    var attributes = memInfo[0].GetCustomAttributes(typeof(UmbracoObjectTypeAttribute),
+                        false);
 
-            var type = typeof(UmbracoObjectTypes);
-            var memInfo = type.GetMember(umbracoObjectType.ToString());
-            var attributes = memInfo[0].GetCustomAttributes(typeof(UmbracoObjectTypeAttribute),
-                false);
+                    if (attributes.Length == 0)
+                        return Guid.Empty;
 
-            if (attributes.Length == 0)
-                return Guid.Empty;
+                    var attribute = ((UmbracoObjectTypeAttribute)attributes[0]);
+                    if (attribute == null)
+                        return Guid.Empty;
 
-            var attribute = ((UmbracoObjectTypeAttribute)attributes[0]);
-            if (attribute == null)
-                return Guid.Empty;
-
-            UmbracoObjectTypeCache.Add(umbracoObjectType, attribute.ObjectId);
-
-            return attribute.ObjectId;
+                    return attribute.ObjectId;
+                });
         }
 
         /// <summary>

src/Umbraco.Core/Persistence/UmbracoDatabase.cs

@@ -16,7 +16,7 @@ namespace Umbraco.Core.Persistence
 	/// can then override any additional execution (such as additional loggging, functionality, etc...) that we need to without breaking compatibility since we'll always be exposing
 	/// this object instead of the base PetaPoco database object.	
 	/// </remarks>
-	public class UmbracoDatabase : Database
+    public class UmbracoDatabase : Database, IDisposeOnRequestEnd
 	{
         private readonly Guid _instanceId = Guid.NewGuid();
 		/// <summary>

src/Umbraco.Core/Umbraco.Core.csproj

@@ -165,6 +165,7 @@
     <Compile Include="Events\SaveEventArgs.cs" />
     <Compile Include="Events\SendToPublishEventArgs.cs" />
     <Compile Include="IApplicationEventHandler.cs" />
+    <Compile Include="IDisposeOnRequestEnd.cs" />
     <Compile Include="IO\ResizedImage.cs" />
     <Compile Include="IO\UmbracoMediaFile.cs" />
     <Compile Include="Media\MediaSubfolderCounter.cs" />