Why doesn't workflowInstance be loaded when the timer expired?

Hi all,

I'm developing a custom persistenceService which is inherited WorkflowPersistenceService.Following code is part of it. The workflowInstance can be unloaded when it is Idle,but the workflowInstance can't be loaded when the timer expired ,and the ReloadWorkflow(object id) method can't be called.Why it happens

protected override bool UnloadOnIdle(Activity activity)
{

return true;
}

protected override void SaveWorkflowInstanceState(Activity rootActivity, bool unlock)
{
// See when the next timer (Delay activity) for this workflow will expire
TimerEventSubscriptionCollection timers = (TimerEventSubscriptionCollection)rootActivity.GetValue(TimerEventSubscriptionCollection.TimerCollectionProperty);
TimerEventSubscription subscription = timers.Peek();
if (subscription != null)
{
// Set a system timer to automatically reload this workflow when it's next timer expires
TimerCallback callback = new TimerCallback(ReloadWorkflow);
TimeSpan timeDifference = subscription.ExpiresAt - DateTime.UtcNow;
Timer timer = new System.Threading.Timer(
callback,
subscription.WorkflowInstanceId,
timeDifference < TimeSpan.Zero TimeSpan.Zero : timeDifference,
new TimeSpan(-1));
}

// Save the workflow
Guid instanceId = (Guid)rootActivity.GetValue(Activity.ActivityContextGuidProperty);
sqlGenerator.SaveWorkflowInstanceState(instanceId, GetDefaultSerializedForm(rootActivity),
(int)GetWorkflowStatus(rootActivity), GetSuspendOrTerminateInfo(rootActivity),
subscription != null subscription.ExpiresAt : DateTime.MaxValue, GetIsBlocked(rootActivity) 1 : 0);
}

private void ReloadWorkflow(object id)
{
// Reload the workflow so that it will continue processing
this.Runtime.GetWorkflow((Guid)id).Load();
}