Question

I have the following multithreaded program:

class Program{
  static void main(){
    (new Thread(DoSomething)).Start();
  }
  static void DoSomething(){
    // Dome something here...
  }
}

A couple of questions:

1. Does the main thread exit after spinning off the child thread?
2. If it does exit and the child thread is a background thread: Is the main process going to exit or will it wait for the background thread to finish?

Answer 1

Typically, if you want to wait for the child thread to complete, you’d add a x.Join(); line (where x is the variable of your thread) wherever you want your main thread to stop and wait for the child to complete.

EDIT: So, yes, the main thread will exit unless one of three cases occurs:
a) the spawned thread completes before the rest of the main thread code (if you add any)

b) You have a condition that waits for the thread to complete (such as the Join statement I mentioned, but there are other ways too).

c) The main thread goes into a semi-infinite loop (such as a game/graphics engine).

In your bare-bones example, it will exit for sure (given your question’s parameters, a background thread).

EDIT2: Sorry, I seem to have dodged your second question (and actually only considered background threads the whole while). If it’s a background thread it will exit as I’ve explained, if it’s foreground then it shouldn’t (though I don’t have much experience with foreground threads, so I can’t say for sure).

So to answer your questions: Yes, the main thread exits. Yes, if the child is specifically a background thread, the process will exit as well.

EDIT3: Final edit, I promise. I just wanted to add some code so that you could prove the answer yourself (which is always nice to be able to do):

static void Main(string[] args)
{
    Thread thready = new Thread(DoSomething);
    thready.IsBackground = true;
    thready.Start();
}

static void DoSomething()
{
    while (true)
    {
        Console.Write("thread's looping \n");
    }
}

By toggling the thready.IsBackground = true; to thready.IsBackground = false; you get a program that runs forever (not exiting until the thread does). Leaving it as true will exit very quickly.