Why volatile and MemoryBarrier do not prevent operations reordering?

Question

If I understand meaning of volatile and MemoryBarrier correctly than the program below has never to be able to show any result.

It catches reordering of write operations

Answer 1

1. If you use MemoryBarrier in writer, why don't you do that in checker? Put Thread.MemoryBarrier(); before int tempA = a;.

2. Calling Thread.MemoryBarrier(); so many times blocks all of the advantages of the method. Call it only once before or after a = 1;.