Question

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?

BigInteger begin = new BigInteger((long)0);
BigInteger end = new BigInteger("340282366920938463463374607431768211456",10);  //2^128
for(begin; begin<end; begin++)
  Console.WriteLine(System.Guid.NewGuid().ToString());

I’m using C#.

Answer 1

Kai, I have provided a program that will do what you want using threads. It is licensed under the following terms: you must pay me $0.0001 per hour per CPU core you run it on. Fees are payable at the end of each calendar month. Please contact me for my paypal account details at your earliest convenience.

using System;
using System.Collections.Generic;
using System.Linq;

namespace GuidCollisionDetector
{
    class Program
    {
        static void Main(string[] args)
        {
            //var reserveSomeRam = new byte[1024 * 1024 * 100];     // This indeed has no effect.

            Console.WriteLine("{0:u} - Building a bigHeapOGuids.", DateTime.Now);
            // Fill up memory with guids.
            var bigHeapOGuids = new HashSet<Guid>();
            try
            {
                do
                {
                    bigHeapOGuids.Add(Guid.NewGuid());
                } while (true);
            }
            catch (OutOfMemoryException)
            {
                // Release the ram we allocated up front.
                // Actually, these are pointless too.
                //GC.KeepAlive(reserveSomeRam);
                //GC.Collect();
            }
            Console.WriteLine("{0:u} - Built bigHeapOGuids, contains {1} of them.", DateTime.Now, bigHeapOGuids.LongCount());


            // Spool up some threads to keep checking if there's a match.
            // Keep running until the heat death of the universe.
            for (long k = 0; k < Int64.MaxValue; k++)
            {
                for (long j = 0; j < Int64.MaxValue; j++)
                {
                    Console.WriteLine("{0:u} - Looking for collisions with {1} thread(s)....", DateTime.Now, Environment.ProcessorCount);
                    System.Threading.Tasks.Parallel.For(0, Int32.MaxValue, (i) =>
                    {
                        if (bigHeapOGuids.Contains(Guid.NewGuid()))
                            throw new ApplicationException("Guids collided! Oh my gosh!");
                    }
                    );
                    Console.WriteLine("{0:u} - That was another {1} attempts without a collision.", DateTime.Now, ((long)Int32.MaxValue) * Environment.ProcessorCount);
                }
            }
            Console.WriteLine("Umm... why hasn't the universe ended yet?");
        }
    }
}

PS: I wanted to try out the Parallel extensions library. That was easy.

And using OutOfMemoryException as control flow just feels wrong.

EDIT

Well, it seems this still attracts votes. So I’ve fixed the GC.KeepAlive() issue. And changed it to run with C# 4.

And to clarify my support terms: support is only available on the 28/Feb/2010. Please use a time machine to make support requests on that day only.

EDIT 2
As always, the GC does a better job than I do at managing memory; any previous attempts at doing it myself were doomed to failure.