Better checking for availability of session

src/Shared/Internal/HttpContextExtensions.cs

@@ -0,0 +1,97 @@
+using System;
+#if !ASP_NET_CORE
+using System.Web;
+#else
+using System.Text;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Features;
+#endif
+using NLog.Common;
+
+namespace NLog.Web.Internal
+{
+    internal static class HttpContextExtensions
+    {
+#if !ASP_NET_CORE
+        internal static HttpRequestBase TryGetRequest(this HttpContextBase context)
+        {
+            try
+            {
+                var request = context?.Request;
+                if (request == null)
+                    InternalLogger.Debug("HttpContext Request Lookup returned null");
+                return request;
+            }
+            catch (HttpException ex)
+            {
+                InternalLogger.Debug(ex, "HttpContext Request Lookup failed.");
+                return null;
+            }
+        }
+#else
+        internal static HttpRequest TryGetRequest(this HttpContext context)
+        {
+            var request = context?.Request;
+            if (request == null)
+                InternalLogger.Debug("HttpContext Request Lookup returned null");
+            return request;
+        }
+#endif
+
+#if ASP_NET_CORE
+        internal static string GetString(this ISession session, string key)
+        {
+            if (!session.TryGetValue(key, out var data))
+            {
+                return null;
+            }
+
+            if (data == null)
+            {
+                return null;
+            }
+
+            if (data.Length == 0)
+            {
+                return string.Empty;
+            }
+
+            return Encoding.UTF8.GetString(data);
+        }
+#endif
+
+#if !ASP_NET_CORE
+        internal static HttpSessionStateBase TryGetSession(this HttpContextBase context)
+        {
+            var session = context?.Session;
+            if (session == null)
+                InternalLogger.Debug("HttpContext Session Lookup returned null");
+            return session;
+        }
+#else
+        internal static ISession TryGetSession(this HttpContext context)
+        {
+            try
+            {
+                if (context?.Features.Get<ISessionFeature>()?.Session != null)
+                {
+                    var session = context?.Session;
+                    if (session == null)
+                        InternalLogger.Debug("HttpContext Session Lookup returned null");
+                    return session;
+                }
+                else
+                {
+                    InternalLogger.Debug("HttpContext Session Feature not available");
+                    return null;
+                }
+            }
+            catch (InvalidOperationException ex)
+            {
+                InternalLogger.Debug(ex, "HttpContext Session Lookup failed.");
+                return null; // System.InvalidOperationException: Session has not been configured for this application or request.
+            }
+        }
+#endif
+    }
+}

src/Shared/LayoutRenderers/AspNetSessionIdLayoutRenderer.cs

@@ -1,7 +1,7 @@
 using System.Text;
-using NLog.Common;
 using NLog.Config;
 using NLog.LayoutRenderers;
+using NLog.Web.Internal;
 
 #if !ASP_NET_CORE
 using System.Web;
@@ -24,16 +24,14 @@ public class AspNetSessionIdLayoutRenderer : AspNetLayoutRendererBase
         protected override void DoAppend(StringBuilder builder, LogEventInfo logEvent)
         {
             var context = HttpContextAccessor.HttpContext;
-            if (context?.Session == null)
-            {
-                InternalLogger.Debug("aspnet-sessionid - HttpContext Session is null");
+            var contextSession = context.TryGetSession();
+            if (contextSession == null)
                 return;
-            }
 
 #if !ASP_NET_CORE
-            builder.Append(context.Session.SessionID);
+            builder.Append(contextSession.SessionID);
 #else
-            builder.Append(context.Session.Id);
+            builder.Append(contextSession.Id);
 #endif
         }
     }

src/Shared/LayoutRenderers/AspNetSessionValueLayoutRenderer.cs

@@ -1,16 +1,11 @@
 using System;
 using System.Globalization;
-using System.Linq;
 using System.Text;
-using NLog.Common;
 using NLog.Config;
 using NLog.LayoutRenderers;
 using NLog.Web.Internal;
 #if !ASP_NET_CORE
 using System.Web;
-#else
-using Microsoft.AspNetCore.Http.Features;
-
 #endif
 
 namespace NLog.Web.LayoutRenderers
@@ -88,23 +83,16 @@ protected override void DoAppend(StringBuilder builder, LogEventInfo logEvent)
             }
 
             var context = HttpContextAccessor.HttpContext;
-            if (context?.Session == null)
-            {
-                InternalLogger.Trace("aspnet-session - HttpContext Session is null");
+            var contextSession = context.TryGetSession();
+            if (contextSession == null)
                 return;
-            }
 
 #if !ASP_NET_CORE
-            var value = PropertyReader.GetValue(Variable, context.Session, (session,key) => session.Count > 0 ? session[key] : null, EvaluateAsNestedProperties);
+            var value = PropertyReader.GetValue(Variable, contextSession, (session,key) => session.Count > 0 ? session[key] : null, EvaluateAsNestedProperties);
 #else
             //because session.get / session.getstring also creating log messages in some cases, this could lead to stackoverflow issues. 
             //We remember on the context.Items that we are looking up a session value so we prevent stackoverflows
-            if (context.Items == null || context.Features.Get<ISessionFeature>()?.Session == null)
-            {
-                return;
-            }
-
-            if (context.Items.Count > 0 && context.Items.ContainsKey(NLogRetrievingSessionValue))
+            if (context.Items == null || (context.Items.Count > 0 && context.Items.ContainsKey(NLogRetrievingSessionValue)))
             {
                 return;
             }
@@ -114,12 +102,7 @@ protected override void DoAppend(StringBuilder builder, LogEventInfo logEvent)
             object value;
             try
             {
-                value = PropertyReader.GetValue(Variable, context.Session, (session, key) => session.GetString(key), EvaluateAsNestedProperties);
-            }
-            catch (Exception ex)
-            {
-                InternalLogger.Warn(ex, "aspnet-session - Retrieving session value failed.");
-                return;
+                value = PropertyReader.GetValue(Variable, contextSession, (session, key) => session.GetString(key), EvaluateAsNestedProperties);
             }
             finally
             {