Question

Suppose i have following code

Rectangle rect = new Rectangle();
lock(rect)
{
  ----
  ---
  ---
}

Can the rect object, or its properties be modified in some other thread, while one thread has acquired the lock, and executing in critical section.

Answer 1

Can the rect object, or its properties be modified in some other
thread, while one thread has acquired the lock, and executing in
critical section.

If all code that accesses that object first attempts to acquire the same lock, then No.

Without the locking code wrapper, then yes another thread can (and at some point will) write to the rect with inconsistent results.