C# lock internal implementation

Question

I know that now in C# lock is implemented in such way:

bool lockWasTaken = false;
var temp = obj;
try 
{ 
     Monitor.Enter(temp, ref lockWasTaken); 
     { 
        //body 
     } 
}
finally 
{ 
     if (lockWasTaken) 
     {
         Monitor.Exit(temp); 
      }
}

Why do we need this: var temp = obj; ?

Answer 1

Simple, What if you changed the variable after the Monitor.Enter called and before Monitor.Exit?

To prevent that it takes up a copy of the variable. Even you can set the value to null also inside the lock statement but still it makes sure that it releases the lock which it taken earlier.

Answer 2

Because obj might be reassigned within the body of the lock code, and the code you've shown has to make sure that it calls Exit on the same object it called Enter for.