suspend and resume a task

Hi,,

            I would like to know how to suspend and resume a task while locking and unlocking the screen. I want to suspend the tasks when the screen is locked and to resume it when the screen is unlocked. I have used IdleDetectionMode.Disabled function and ApplicationIdleModeHelper.Current.HasUserAgreedToRunUnderLock to check the screen lock condition. But it is not working. I am not able to stop the task. Please help me regarding this issue. It would be nice if u could send some useful resources ,classes or links based on this.
Waiting for ur reply.

thanks,
Aswathy.