Why Do Multiple Random.Next Instances Return Identical Values?

Understanding Identical Random Number Generation in C#

When generating random numbers within a loop using multiple instances of a class, you might observe that Random.Next() produces the same values repeatedly. This behavior stems from the way the Random class functions.

The Random class relies on a seed value to initiate its pseudo-random number sequence. If multiple Random instances are created in quick succession, they often receive very similar seed values, resulting in identical output sequences.

The Solution: A Shared Random Instance

To overcome this, instead of creating a new Random instance for each class object, create a single Random instance and share it among all instances of your class. This ensures that each instance draws from the same, evolving sequence of random numbers.

Here's an improved example demonstrating this approach:

public class A
{
    private readonly Random _rnd;

    public A(Random rnd)
    {
        _rnd = rnd;
    }

    public void Count()
    {
        int r1 = _rnd.Next(-1, 1); // Note: Next(-1, 1) will only return -1 or 0.  To get -1, 0, or 1, use Next(-1, 2)
        int r2 = _rnd.Next(-1, 2); // Corrected to include 1
    }
}

public class B
{
    private readonly List<A> _listOfA = new List<A>();
    private readonly Random _rnd = new Random();

    public B()
    {
        // Populate _listOfA...  (Example below)
        for (int i = 0; i < 10; i++)
        {
            _listOfA.Add(new A(_rnd));
        }
    }

    public void Process()
    {
        foreach (var aClass in _listOfA)
        {
            aClass.Count();
        }
    }
}

By injecting the shared _rnd instance into the A class constructor, each A instance uses the same random number generator, guaranteeing distinct random numbers across all instances. Note the correction in Next(-1,2) to include 1 in the possible outputs.

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