How to monitor if a Thread completed?

What you want to do is call the method (your longrunningtask) using an asynchronous delegate. This is better than normal multithreading (using the threadpool for example) in that one can get exceptions thrown on the original thread back to the caller when the caller re-unites with the delegate (in EndInvoke). The caller can then rethrow the exception (something not possible in normal threading usage).

This example assumes that your LongRunningProcess returns a string (but that can be changed in the delegate definition).

Also - if you are doing this from a UI control (a WinForm for example) - a BackgroundWorker is the way to go. It has built in cancellation and progress notification capabilities - and is extremely straightforward to use. It keeps the UI responsive while delegating the background task.

delegate string LongRunningTask(CancellationToken ct);

static void MainMethod() {

 CancellationToken ct = tokenSource2.Token;
// Instantiate delegates with  LongRunningTask's signature:  
LongRunningTask task1 = LongRunningProcess(ct);

// Start the invocation
 IAsyncResult cookie = task1.BeginInvoke (ct, null, null);
 
// Can do something else here if needed (PerformOtherTask())
// PerformOtherTask();

 // Get the results of the LongRunningProcess, waiting for completion if necessary.
 // Here is where any exceptions will be thrown:

 try
  {
    string s1 = task1.EndInvoke (cookie);
  }
 catch(Exception e)
  {
    throw;
  }

}

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