Notify when thread is complete, without locking calling thread

There are two slightly different kinds of requirement here:

• Execute a callback once the long-running task has completed
• Execute a callback once the thread in which the long-running task was running has completed.

If you're happy with the first of these, the simplest approach is to create a compound task of "the original long-running task, and the callback", basically. You can even do this just using the way that multicast delegates work:

ThreadStart starter = myLongRunningTask;
starter += () => {
    // Do what you want in the callback
};
Thread thread = new Thread(starter) { IsBackground = true };
thread.Start();

That's very vanilla, and the callback won't be fired if the thread is aborted or throws an exception. You could wrap it up in a class with either multiple callbacks, or a callback which specifies the status (aborted, threw an exception etc) and handles that by wrapping the original delegate, calling it in a method with a try/catch block and executing the callback appropriately.

Unless you take any special action, the callback will be executed in the background thread, so you'll need to use Control.BeginInvoke (or whatever) to marshal back to the UI thread.

I absolutely understand your requirements, but you've missed one crucial thing: do you really need to wait for the end of that thread synchronously? Or maybe you just need to execute the "finalizer" after thread's end is detected?

In the latter case, simply wrap the call to myLongRunningTask into another method:

void surrogateThreadRoutine() {
    // try{ ..

    mytask();

    // finally { ..
    ..all 'finalization'.. or i.e. raising some Event that you'll handle elsewhere
}

and use it as the thread's routine. That way, you'll know that the finalization will occur at the thread's and, just after the end of the actual job.

However, of course, if you're with some UI or other schedulers, the "finalization" will now run on yours thread, not on the "normal threads" of your UI or comms framework. You will need to ensure that all resources are external to your thread-task are properly guarded or synchronized, or else you'll probably clash with other application threads.

For instance, in WinForms, before you touch any UI things from the finalizer, you will need the Control.InvokeRequired (surely=true) and Control.BeginInvoke/Invoke to bounce the context back to the UI thread.

For instance, in WPF, before you touch any UI things from the finalizer, you will need the Dispatcher.BeginInvoke..

Or, if the clash could occur with any threads you control, simple proper lock() could be enough. etc.