Random number with Probabilities in C#

I have converted this Java program into a C# program.

using System;
using System.Collections.Generic;

namespace RandomNumberWith_Distribution__Test
{
    public class DistributedRandomNumberGenerator
    {

        private Dictionary<Int32, Double> distribution;
        private double distSum;

        public DistributedRandomNumberGenerator()
        {
            distribution = new Dictionary<Int32, Double>();
        }

        public void addNumber(int val, double dist)
        {
            distribution.Add(val, dist);// are these two
            distSum += dist;            // lines correctly translated?
        }

        public int getDistributedRandomNumber()
        {
            double rand = new Random().NextDouble();//generate a double random number
            double ratio = 1.0f / distSum;//why is ratio needed?
            double tempDist = 0;

            foreach (Int32 i in distribution.Keys)
            {
                tempDist += distribution[i];

                if (rand / ratio <= tempDist)//what does "rand/ratio" signify? What does this comparison achieve?
                {
                    return i;
                }
            }
            return 0;
        }
    }

    public class MainClass
    {
        public static void Main(String[] args)
        {
            DistributedRandomNumberGenerator drng = new DistributedRandomNumberGenerator();
            drng.addNumber(1, 0.2d);
            drng.addNumber(2, 0.3d);
            drng.addNumber(3, 0.5d);

            //================= 
            // start simulation
            int testCount = 1000000;
            Dictionary<Int32, Double> test = new Dictionary<Int32, Double>();

            for (int i = 0; i < testCount; i++)
            {
                int random = drng.getDistributedRandomNumber(); 

                if (test.ContainsKey(random)) 
                {
                    double prob = test[random];   // are these
                    prob = prob + 1.0 / testCount;// three lines
                    test[random] = prob;          // correctly translated?
                }
                else
                {
                    test.Add(random, 1.0 / testCount);// is this line correctly translated?
                }
            }

            foreach (var item in test.Keys)
            {
                Console.WriteLine($"{item}, {test[item]}");
            }

            Console.ReadLine();
        }
    }
}

I have several questions:

1. Can you check if the marked-by-comment lines are correct or need explanation?
2. Why doesn't getDistributedRandomNumber() check if the sum of the distribution 1 before proceeding to further calculations?