Example about generating random number using the alias method in C#

    public static class RandomExtensions
    {
        public static int GetAlias(this Random rnd, IEnumerable<int> probs)
        {
            int random = rnd.Next(probs.Sum());
            int sum = 0;
            int idx = 0;
            foreach (var p in probs)
            {
                sum += p;
                if (sum >= random)
                    break;
                idx++;
            }

            return idx;
        } // GetAlias
    }

We pass the method a list of int containing the probability we want for that index.

Test program

    class Program
    {               
        class Animal 
        {
            public Animal(string name) {
                Name = name;
            }
            public string Name { get; set; }
            public int Count { get; set; }
        } 

        static void Main(string[] args)
        {
            List<Animal> values = new List<Animal>
            {
                new Animal("Cat"),
                new Animal("Dog"),
                new Animal("Sheep"),
                new Animal("Cow"),
                new Animal("Turtle"),
            };

            int[] probs = new int[] { 10, 50, 10, 10, 20 };
            Random rnd = new Random();
            for (int i=0; i < 100; i++) {
                int number = rnd.GetAlias(probs);
                values[number].Count++;
            }

            foreach (var a in values) {
                Console.WriteLine($"{string.Format("{0,-10}", a.Name)}: {string.Format("{0,2}", a.Count)}");
            }
        }
    }

Test program output

Cat       :  6
Dog       : 49
Sheep     : 12
Cow       : 11
Turtle    : 22