Two different random number generators giving the same answer?

Question

    static void Main(string[] args)
    {

        //random number gen
        Console.WriteLine("Array Random Number:");
        randomGenA();
        Console.WriteLine("------------------");

        //LIST: random movie picker...
        Console.WriteLine("List Random Number:");
        randomGenB();
        Console.WriteLine("------------------");

        Console.ReadLine();
    }

    static void randomGenA()
    {
        Random randomA = new Random();
        int randomNumA = randomA.Next(51);

        Console.WriteLine(randomNumA);

    }
    static void randomGenB()
    {
        Random randomB = new Random();
        int randomNumB = randomB.Next(0,51);

        Console.WriteLine(randomNum);
    }
 }

I wanted them both to produce two different random numbers but instead I keep getting the same random number from both of them. Why does it do this?

The time difference between the instantiations of these two `Random` objects is minimal, causing them to have the same seed. Introduce a delay (please don't) or share the `Random` instance. — Jeroen Vannevel, Sep 02 '15 at 20:28
Possible duplicate of [this question](http://stackoverflow.com/questions/4060961/seeding-a-pseudo-random-number-generator-in-c-sharp) — themattkellyshow, Sep 02 '15 at 20:30
add System.Threading.Thread.Sleep(50); between your method call — Abin, Sep 02 '15 at 20:32
create one `Random randomB = new Random();` in class and use it in both methods. — M.kazem Akhgary, Sep 02 '15 at 20:32
Many good solutions here, the one I have not seen yet: use a random number from first generator as seed for second. Easiest one though is probably the shared instance. — Andreas Reiff, Sep 02 '15 at 20:54

lintmouse · Accepted Answer · 2015-09-02T20:45:53.507

Declare a class level random and use it in your methods:

private static Random _random = new Random();

Your methods would look like:

static void randomGenA()
{
    int randomNumA = _random.Next(51);

    Console.WriteLine(randomNumA);

}
static void randomGenB()
{
    int randomNumB = _random.Next(0,51);

    Console.WriteLine(randomNum);
}

Check this out for further reading: http://www.dotnetperls.com/random

score 0 · Answer 2 · answered Sep 02 '15 at 20:28

As per the documentation:

https://msdn.microsoft.com/en-us/library/system.random%28v=vs.110%29.aspx

Two Randoms with the same seed will provide the same sequence of numbers. Simply don't use the same seed. If you don't specify a seed, it uses the system time. Wait some time between the two instantiations and it will use two different system times.

Two different random number generators giving the same answer?

2 Answers2