UnityMoqTesting

RandomUtils.cs

using System.Collections.Generic;
using UnityEngine;

public interface IRandomGenerator
{
    float GenerateValue();
    int GenerateInteger(int min, int max);
}

internal class UnityRandomGenerator: IRandomGenerator {
    public float GenerateValue()
    {
        return Random.value;
    }

    public int GenerateInteger(int min, int max)
    {
        return Random.Range(min, max);
    }
}

public class RandomUtils { 
    private static RandomUtils instance = null;
    public static RandomUtils Instance
    {
        get {
            if (instance == null)
            {
                instance = new RandomUtils();
            } 
            return instance;
        }
    }
    private readonly IRandomGenerator generator = null;
    
    public RandomUtils(IRandomGenerator generator)
    {
        this.generator = generator;
    }

    internal RandomUtils()
    {
        generator = new UnityRandomGenerator();
    }
    
    public T Choice<T>(IList<T> list)
    {
        var index = generator.GenerateInteger(0, list.Count - 1);
        return list[index];
    }
}

RandomUtilsTests.cs

using UnityEngine;
using System.Collections.Generic;
using NUnit.Framework;
using Moq;

public class RandomUtilsTests {
    [Test]
    public void TestChoice()
    {
        var generator = new Mock<IRandomGenerator>(); 
        generator.Setup(m => m.GenerateInteger(It.Is<int>(i => i == 0), It.Is<int>(i => i == 3))).Returns(1);

        var list = new List<int> { 1, 4, 9, 16 };
        var utils = new RandomUtils(generator.Object);
        Assert.AreEqual(utils.Choice(list), 4);
    }
}