Calculate how much time taken to complete a task

To calculate or just to measure? I think — to measure.

More accurate than System.DateTime is the class System.Diagnostics.Stopwatch:
http://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch.aspx[^].

This is the best way to measure performance and related time interval values. The class documentation fully describes the use and the features.

—SA

Well basically you need to store just few pieces of information

• The time the person started to answer questions
• The fixed amount of time that's given to answer the question

To get the current time, you can use DateTime.Now[^] property. Using this property you can record the time when answering started and you can also use it when you calculate how much time has been elapsed. To get the remaining time you just subtract the elapsed time from the allowed time.

In order to update your UI to show the remaining time you can use timer as already said or an instance of a BackgroundWorker[^] class.

Just remember that you cannot update your UI from another thread so you need to check that the calling thread is the same as the UI thread. In order to do this, you can use the CheckAccess[^] method of the Dispatcher.

Not too many codes to do it..the answer of Christian Graus is absolutely right..i add a little bit with it.Set timer for your application..and increase it for every second in timer_tick event.I mean second++..By it you will measure how much time taken to answer question,then as mentioned earlier by Christian Graus,subtract it from fixed time.