What is proper way to prevent two separate threads from running at same time?

I have two functions, one is timer call back function, and the other is a function which is called by a running thread, it is not the thread start function itself, this second function is called by another thread.

(One is a timer thread)

If I hit timer_Elapsed function first, I want to block on DoStuff until function exits

If I hit DoStuff function first I want to block on timer_Elapsed until function exits

What is the proper as well as efficient way to do this.

private void timer_Elapsed(object sender, ElapsedEventArgs e)

{

...do timer stuff....

}

{thread started somewhere else....at some point calls DoStuff() }

void DoStuff()

{

...do my stuff...

}