Locking Variables when Threading in C#

I have a C# program, where I'm spawning a thread to do some calculations. I'm then adding the result of the calculations to a Queue, and from the main thread, I'm constantly checking to see if the Queue has length more than 0. If it does, then the result of the calculation is de-queued and used elsewhere.

I've read that I should lock the queue when accessing it from either thread because it may cause problems if both threads are accessing it at the same time. But should I lock it whenever I do ANYTHING with the Queue, or only when en-queuing/de-queuing?

E.g.

// In main thread

lock (meshDataQueue) {
  if (meshDataQueue.Count > 0)
  {
    constructMesh(meshDataQueue.dequeue())
  }
}

vs.

if (meshDataQueue.Count > 0) {
  lock (meshDataQueue)
  {
    constructMesh(meshDataQueue.dequeue())
  }
}