internal static void PostInternal(WorkItemCallback callback) {
     if (_useQueueUserWorkItem) {
         ThreadPool.QueueUserWorkItem(_onQueueUserWorkItemCompletion, callback);
     }
     else {
         WrappedWorkItemCallback w = new WrappedWorkItemCallback(callback);
         w.Post();
     }
 }
 internal static void PostInternal(WorkItemCallback callback)
 {
     if (_useQueueUserWorkItem)
     {
         ThreadPool.QueueUserWorkItem(_onQueueUserWorkItemCompletion, callback);
     }
     else
     {
         WrappedWorkItemCallback w = new WrappedWorkItemCallback(callback);
         w.Post();
     }
 }