title: Simple proof that GUID is not unique
body: <p>I'd like to prove that a GUID is not unique in a simple test program.
  I expected the following code to run for hours, but it's not working. How can I make it work?</p>
  
  <pre><code>BigInteger begin = new BigInteger((long)0);
  BigInteger end = new BigInteger("340282366920938463463374607431768211456",10);  //2^128
  for(begin; begin&lt;end; begin++)
    Console.WriteLine(System.Guid.NewGuid().ToString());
  </code></pre>
  
  <p>I'm using C#.</p>