How do I prevent my threads from obtaining the same random number?

Question

I'm trying to make 3 threads generate random numbers, but they keep picking up the same number. What am I doing wrong with the threads?

Here is my "Driver" essentially:

public int[] GetOffers()
    {
        _3InsuranceCompanies guys = new _3InsuranceCompanies();

        Thread thread1 = new Thread(new ThreadStart(guys.sellers));
        Thread thread2 = new Thread(new ThreadStart(guys.sellers));
        Thread thread3 = new Thread(new ThreadStart(guys.sellers));

        thread1.Start();
        thread2.Start();
        thread3.Start();


        int[] offers = new int[3];
        offers[0] = guys.insurance_sellers[0];
        offers[1] = guys.insurance_sellers[1];
        offers[2] = guys.insurance_sellers[2];


        return offers;
    }

Here is the class the threads are using as a start method:

 public class _3InsuranceCompanies
{

    public int[] insurance_sellers = new int[3];
    public _3InsuranceCompanies()
    {
        for(int x = 0; x < 3; x++)
        {
            insurance_sellers[x] = new Random().Next(60, 200);
        }
    }
    public void sellers()
    {
        Random rnd = new Random();
        int price;

        // simulated negotiation. some threads might be good at negotiating so price will reduce
        // other threads might be bad at negotiating so price can possible go higher than before

        Monitor.TryEnter(insurance_sellers);
        for (int x = 0; x < 3; x++)
        {
            price = rnd.Next(70, 200);
            if (Math.Abs(insurance_sellers[0] - price) < 30)
                insurance_sellers[0] = price;
            if (Math.Abs(insurance_sellers[1] - price) < 30)
                insurance_sellers[1] = price;
            if (Math.Abs(insurance_sellers[2] - price) < 30)
                insurance_sellers[2] = price;
        }
        Monitor.Exit(insurance_sellers);
    }
}

tymtam · Answer 1 · 2019-10-28T01:59:52.187

Move new Random out of the method and make static.

static Random rnd = new Random();

public _3InsuranceCompanies()
    {
        for(int x = 0; x < 3; x++)
        {
            insurance_sellers[x] = rnd.Next(60, 200);
        }
    }

What's happening?

The following passage is from Instantiating the random number generator from Random's documenatation

If the same seed is used for separate Random objects, they will generate the same series of random numbers. This can be useful for creating a test suite that processes random values, or for replaying games that derive their data from random numbers.

Here's a little example:

var count = 10;

var randoms = new int[count];
for (int x = 0; x < count; x++) { randoms[x] = new Random().Next(60, 200); }
Console.WriteLine($"With many new Random()s: {string.Join(",", randoms)}");

var rnd = new Random();
var randoms2 = new int[count];
for (int x = 0; x < count; x++) { randoms2[x] = rnd.Next(60, 200); }

Console.WriteLine($"With one Random: {string.Join(",", randoms2)}");

Output

With many new Random()s: 70,70,70,70,70,70,70,70,70,70
With one Random: 70,89,114,197,89,117,176,64,61,103

How do I prevent my threads from obtaining the same random number?

1 Answers1

What's happening?