Thread-safe Item Collections for Concurrent mode

src/CosmosDBSessionStateProviderAsync/CosmosDBSessionStateProviderAsync.cs

@@ -611,7 +611,7 @@ public override SessionStateStoreData CreateNewStoreData(HttpContextBase context
                 staticObjects = GetSessionStaticObjects(context.ApplicationInstance.Context);
             }
 
-            return new SessionStateStoreData(new SessionStateItemCollection(), staticObjects, timeout);
+            return new SessionStateStoreData(CreateItemCollection(), staticObjects, timeout);
         }
 
         /// <inheritdoc />
@@ -628,7 +628,7 @@ public override async Task CreateUninitializedItemAsync(
 
             string encodedBuf;
 
-            var item = new SessionStateStoreData(new SessionStateItemCollection(),
+            var item = new SessionStateStoreData(CreateItemCollection(),
                         GetSessionStaticObjects(context.ApplicationInstance.Context),
                         timeout);
 
@@ -1038,6 +1038,28 @@ private static string GetEncodedStringFromMemoryStream(MemoryStream s)
             return Convert.ToBase64String(bytes.Array, bytes.Offset, bytes.Count);
         }
 
+        private static ISessionStateItemCollection CreateItemCollection()
+        {
+            return SessionStateModuleAsync.AllowConcurrentRequestsPerSession ?
+                new ConcurrentSessionStateItemCollection() as ISessionStateItemCollection :
+                new SessionStateItemCollection() as ISessionStateItemCollection;
+        }
+
+        private static void SerializeItemCollection(ISessionStateItemCollection items, BinaryWriter writer)
+        {
+            if (items is ConcurrentSessionStateItemCollection concurrentItems)
+                concurrentItems.Serialize(writer);
+            else if (items is SessionStateItemCollection defaultItems)
+                defaultItems.Serialize(writer);
+        }
+
+        private static ISessionStateItemCollection DeserializeItemCollection(BinaryReader reader)
+        {
+            return SessionStateModuleAsync.AllowConcurrentRequestsPerSession ?
+                ConcurrentSessionStateItemCollection.Deserialize(reader) as ISessionStateItemCollection :
+                SessionStateItemCollection.Deserialize(reader) as ISessionStateItemCollection;
+        }
+
         private static void Serialize(SessionStateStoreData item, Stream stream)
         {
             bool hasItems = true;
@@ -1060,7 +1082,7 @@ private static void Serialize(SessionStateStoreData item, Stream stream)
 
             if (hasItems)
             {
-                ((SessionStateItemCollection)item.Items).Serialize(writer);
+                SerializeItemCollection(item.Items, writer);
             }
 
             if (hasStaticObjects)
@@ -1089,7 +1111,7 @@ internal static SessionStateStoreData DeserializeStoreData(HttpContextBase conte
         private static SessionStateStoreData Deserialize(HttpContextBase context, Stream stream)
         {
             int timeout;
-            SessionStateItemCollection sessionItems;
+            ISessionStateItemCollection sessionItems;
             bool hasItems;
             bool hasStaticObjects;
             HttpStaticObjectsCollection staticObjects;
@@ -1106,11 +1128,11 @@ private static SessionStateStoreData Deserialize(HttpContextBase context, Stream
 
                 if (hasItems)
                 {
-                    sessionItems = SessionStateItemCollection.Deserialize(reader);
+                    sessionItems = DeserializeItemCollection(reader);
                 }
                 else
                 {
-                    sessionItems = new SessionStateItemCollection();
+                    sessionItems = CreateItemCollection();
                 }
 
                 if (hasStaticObjects)

src/SessionStateModule/InProcSessionStateStoreAsync.cs

@@ -437,7 +437,14 @@ private SessionStateStoreData CreateLegitStoreData(
         {
             if (sessionItems == null)
             {
-                sessionItems = new SessionStateItemCollection();
+                if (SessionStateModuleAsync.AllowConcurrentRequestsPerSession)
+                {
+                    sessionItems = new ConcurrentNonSerializingSessionStateItemCollection();
+                }
+                else
+                {
+                    sessionItems = new SessionStateItemCollection();
+                }
             }
 
             if (staticObjects == null && context != null)

src/SessionStateModule/Microsoft.AspNet.SessionState.SessionStateModule.csproj

@@ -66,8 +66,10 @@
     </Compile>
     <Compile Include="SessionEventSource.cs" />
     <Compile Include="SessionOnEndTarget.cs" />
+    <Compile Include="SessionStateItemCollections.cs" />
     <Compile Include="SessionStateModuleAsync.cs" />
     <Compile Include="SessionStateStoreProviderAsyncBase.cs" />
+    <Compile Include="StateSerializationUtil.cs" />
     <Compile Include="TaskAsyncHelper.cs" />
   </ItemGroup>
   <ItemGroup>

src/SessionStateModule/Resources/SR.resx

@@ -117,6 +117,9 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <data name="Cant_serialize_session_state" xml:space="preserve">
+    <value>Unable to serialize the session state. For out-of-proc session stores, ASP.NET will serialize the session state objects, and as a result non-serializable objects or MarshalByRef objects are not permitted.</value>
+  </data>
   <data name="Error_Occured_Reading_Config_Secion" xml:space="preserve">
     <value>Error occured when reading config secion '{0}'.</value>
   </data>
@@ -126,6 +129,9 @@
   <data name="Invalid_session_custom_provider" xml:space="preserve">
     <value>The custom session state store provider name '{0}' is invalid.</value>
   </data>
+  <data name="Invalid_session_state" xml:space="preserve">
+    <value>The session state information is invalid and might be corrupted.</value>
+  </data>
   <data name="Missing_session_custom_provider" xml:space="preserve">
     <value>The custom session state store provider '{0}' is not found.</value>
   </data>