Question

Possible Duplicate:
Why does this Random Number Generator not random?

I have this test program:

static void Main(string[] args)
{
    var randomNumbers = new Dictionary<int, int>();
    foreach (var s in Enumerable.Range(1, 500))
    {
        var rand = Rand5();
        if (!randomNumbers.ContainsKey(rand))
            randomNumbers.Add(rand, 1);
        else
            randomNumbers[rand] += 1;
    }

    randomNumbers
        .ToList()
        .ForEach(x => Console.WriteLine("{0}: {1}", x.Key, x.Value));
    Console.ReadLine();
}

static int Rand5()
{
    System.Threading.Thread.Sleep(1);
    return new Random().Next(1, 6);
}

If I comment out System.Threading.Thread.Sleep(1);, I get

5: 500

But if I uncomment that line, I do get random numbers.

2: 87
4: 94
1: 116
5: 108
3: 95

Why does the line of code matter? Thanks!

Answer 1

As others have said, new Random() seeds the random number generator from the current system time.

I have an article describing this in more detail, including solutions to the problem, which you may find useful. Basically you want to use the same instance of Random multiple times – but observing that it’s not thread-safe.