Question

Why exceptions thrown within a task are silent exception and you never know if a certain exception has been thrown

try
{

 Task task = new Task(
  () => {
          throw null;
        }
        );
  task.Start();
 }
 catch
 {
  Console.WriteLine("Exception");
 }  

the program run successfully in a complete silence!
where the behavior of threads is different

try
{

 Thread thread = new Thread(
  () => {
          throw null;
        }
        );
  thread .Start();
 }
 catch
 {
  Console.WriteLine("Exception");
 }

a null pointer exception will be thrown in this case.
What is the difference?

Answer 1

The behaviour of that scenario depends on what framework you have; in 4.0, you actually need to be careful – if you don’t handle TaskScheduler.UnobservedTaskException, it will error later when it gets collected/finalized, and will kill your process.

TaskScheduler.UnobservedTaskException += (sender, args) =>
{
    Trace.WriteLine(args.Exception.Message); // somebody forgot to check!
    args.SetObserved();
};

This changes in 4.5, IIRC.

To check the outcome of a Task that might fail, you could register a continuation – i.e. call ContinueWith and check the result’s exception. Alternatively, accessing the .Result of the task (which would also do an implicit Wait()) will re-surface the exception that happened. It is good to observe the result of a task, as that clears the finalization flag, meaning it can be collected more cheaply.