I want to avoid using fields when sharing data between 2 threads.
Consider 2 classes – user and handler.
User constructs handler and tells it to change a user.flag variable after 1000ms.

There could be many users with various flags. I can of course use an event or action or callback but wont it be a lot nicer if I could pass flag by ref and simply change its value

(NOTE – user does not need to know when exactly flag value changed! It only needs to have flag == true at any arbitrary point of time after 1000ms user checks the flag)

I would write something like this (without using unsafe):

bool userFlag;
void UserInitHandler()
{
  var handler = new Handler(1000)
  handler.Start(ref userFlag);
}

class Handler
{
  ...
  void Start(ref bool flag)
  {
    ThreadPool.QueueUserWorkItem((o) => 
    {
      Thread.Sleep(delay);
      flag = true;
    }      
  }
}