Question

i’m new at C # and i’m creating a racetrack simulator, but i’m currently encountering some issues when i run my code. I have an array of four Greyhound objects, and if I call Greyhound.Run on my form1.cs and i don’t have a “MessageBox.Show(“Distance” + distance)” on my Run method, which shows me how many pixels each greyhound is supposed to move, all of the greyhounds end up moving the same distance. I don’t understand why this is happening

namespace Race
{
    class Greyhound
    {
        public int StartingPosition;
        public int RacetrackLength;
        public PictureBox MyPictureBox = null;
        public int Location = 0;
        public Random Randomizer;

        public bool Run()
        {
            Point p = MyPictureBox.Location;
            if (p.X + MyPictureBox.Width >= RacetrackLength)
            {
                //TakeStartingPostion();
                return true;

            }
            else
            {
                Randomizer = new Random();
                int distance = Randomizer.Next(100);
                MessageBox.Show("Distance is " + distance);
                p.X += distance;
                MyPictureBox.Location = p;
                return false;
            }

        }

        public void TakeStartingPostion()
        {
            Point P = MyPictureBox.Location;
            P.X = StartingPosition;
            MyPictureBox.Location = P;
        }
    }
}


namespace Race
{
    public partial class Form1 : Form
    {
        Guy[] guys = new Guy[3];
        Greyhound[] hounds = new Greyhound[4];

        public Form1()
        {
            InitializeComponent();

            hounds[0] = new Greyhound()
            { 
                StartingPosition = 12,
                MyPictureBox = GreyHound1,
                RacetrackLength = 636
            };

            hounds[1] = new Greyhound()
            { 
                StartingPosition = 12,
                MyPictureBox = GreyHound2,
                RacetrackLength = 636
            };

            hounds[2] = new Greyhound()
            { 
                StartingPosition = 12,
                MyPictureBox = GreyHound3,
                RacetrackLength = 636
            };

            hounds[3] = new Greyhound()
            { 
                StartingPosition = 12,
                MyPictureBox = GreyHound4,
                RacetrackLength = 636
            };
        }

        private void button2_Click(object sender, EventArgs e)
        {
            for (int i = 0; i < hounds.Length; i++)
            {

                 hounds[i].Run();
            } 

        }
    }
}

Answer 1

It’s because you’re calling new Random() each time the Run() method hits the else block. The default Random constructor initializes the pseudorandom number generator based on the current time. When there’s no interruption, all 4 method run at “the same time”, so they spit out the same random numbers. To fix this, either create only a single Random, or else use a different seed for each one (by using the Random constructor that takes a seed as a parameter.)

Something like this would work:

public class Greyhound
{
    public static Random randomizer = new Random();

    // ... In the run method ...
    int distance = Greyhound.randomizer.Next(100);

}

Update: As Groo points out, if you were actually calling Next() from multiple threads, the code I’ve shown isn’t thread-safe. While that’s not the case with your code, it’s a good idea to be aware of issues like this earlier rather than later. The general solution to this (potential) problem is to surround the call to Next() with a lock, like so:

// ... After the declaration of randomizer ...
private static object randomLock = new object();

// ... New Next() call...
lock (randomLock)
    Greyhound.randomizer.Next(100);