Why Does Random.Next Produce Identical Values in Consecutive Loop Iterations?

Random Number Generation Issues in Loops

This article addresses a common problem encountered when using Random.Next() within loops: consecutive iterations producing identical random numbers. The issue often manifests when multiple Random objects are instantiated in quick succession.

Problem Scenario:

Consider the following code snippet:

<code class="language-csharp">class A
{
    Random rnd = new Random();
    public void Count()
    {
        int r1 = rnd.Next(-1, 1);
        int r2 = rnd.Next(-1, 1);
        // ... further code using r1 and r2 ...
    }
}

class B
{
    List<A> listOfA = new List<A>();
    public B()
    {
        // ... populate listOfA ...
        foreach (A instance in listOfA)
        {
            instance.Count();
        }
    }
}</code>

If listOfA contains multiple instances of class A, each A object creates its own Random instance. Due to the high-speed instantiation, these Random objects might receive the same seed value (often derived from the system clock), leading to identical random number sequences.

Root Cause:

The problem stems from the default Random constructor's reliance on the system clock for seeding. If multiple Random objects are created within a very short timeframe, they receive virtually identical seed values, resulting in identical pseudo-random number generation.

Solution: Single Random Instance

The solution is straightforward: create a single Random instance and reuse it across all instances of class A. This ensures that a unique, non-repeating sequence of random numbers is generated.

Here's the corrected code:

<code class="language-csharp">class A
{
    private Random rnd;
    public A(Random rnd)
    {
        this.rnd = rnd;
    }

    public void Count()
    {
        int r1 = rnd.Next(-1, 1);
        int r2 = rnd.Next(-1, 1);
        // ... further code using r1 and r2 ...
    }
}

class B
{
    Random rnd = new Random();
    List<A> listOfA = new List<A>();
    public B()
    {
        // ... populate listOfA, passing rnd to the constructor ...
        foreach (A instance in listOfA)
        {
            instance.Count();
        }
    }
}</code>

By injecting the single Random instance (rnd) into the A class constructor, we guarantee that all calls to rnd.Next() draw from the same, properly seeded sequence. This effectively eliminates the generation of duplicate random numbers.

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