Fixed mono fix by checking if the Enum value exists in the runtime.

src/NzbDrone.Core/Messaging/Commands/CommandExecutor.cs

@@ -81,11 +81,18 @@ namespace NzbDrone.Core.Messaging.Commands
             //       For now we use TaskCreationOptions 0x10, which is actually .net 4.5 HideScheduler.
             //       This will detach the scheduler from the thread, causing new Task creating in the command to be executed on the ThreadPool, avoiding a deadlock.
             //       Please note that the issue only shows itself on mono because since Microsoft .net implementation supports Task inlining on WaitAll.
-            _taskFactory.StartNew(() => ExecuteCommand<TCommand>(command)
+            if (Enum.IsDefined(typeof(TaskCreationOptions), (TaskCreationOptions)0x10))
-                , TaskCreationOptions.PreferFairness)
+            {
-//                  This breaks on systems that don't have .Net 4.5 installed (but works fine when it does, even though we are targetting 4.0)
+                _taskFactory.StartNew(() => ExecuteCommand<TCommand>(command)
-//                , TaskCreationOptions.PreferFairness | (TaskCreationOptions)0x10)
+                    , TaskCreationOptions.PreferFairness | (TaskCreationOptions)0x10)
-                .LogExceptions();
+                    .LogExceptions();
+            }
+            else
+            {
+                _taskFactory.StartNew(() => ExecuteCommand<TCommand>(command)
+                    , TaskCreationOptions.PreferFairness)
+                    .LogExceptions();
+            }
 
             return command;
         }