Cancel an already executed Task

Question

Hi,

My question is How do you cancel an already executed task.

 

Lets say I have 10 tasks and I have put these tasks in a timer based "async" method. Since it is a timer based application, these tasks will be executed again and again. But, I don't want this to happen. If Task 1 is already finished then it should not be executed again.

Can anyone help me out in this regard with an example.