EventDemo

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;


namespace ProgDemo01
{
    class Program
    {

        static void Main(string[] args)
        {

            var primeNumberCounter1 = new PrimeNumberCounter(10000, 50000);
            primeNumberCounter1.StartCalculation += primeNumberCounter1_StartCalculation;
            primeNumberCounter1.FinishCalculation += primeNumberCounter1_FinishCalculation;
            primeNumberCounter1.RunCounter();

        }

        private static void primeNumberCounter1_FinishCalculation(PrimeNumberCounter pm, EventArgs args)
        {
            Console.WriteLine("Обчислення завершено");
            Console.WriteLine(pm.Counter);
        }

        private static void primeNumberCounter1_StartCalculation(PrimeNumberCounter pm, EventArgs args)
        {
            Console.WriteLine("Обчислення розпочато");
        }
    }

    //Обчислити кількість простих чисел від [A;B]

    public class PrimeNumberCounter
    {
        public delegate void CalculationHandler (PrimeNumberCounter pm, EventArgs args);
        public event CalculationHandler StartCalculation;
        public event CalculationHandler FinishCalculation;

        public int StartNumber { get; set; }
        public int FinishNumber { get; set; }
        public int Counter { get; set; }

        public PrimeNumberCounter()
        {
            StartNumber = 0;
            FinishNumber = 0;
            Counter = 0;
        }

        public PrimeNumberCounter(int start, int finish)
        {
            StartNumber = start;
            FinishNumber = finish;
            Counter = 0;
        }

        public void RunCounter()
        {
            // StartCalculation
            if (StartCalculation != null)
            {
                StartCalculation(this, new EventArgs());
            }

            for (int i = StartNumber; i < FinishNumber; i++)
            {
                if (isPrime(i)) Counter++;
            }
            // FinishCalculation
            if (FinishCalculation != null)
            {
                FinishCalculation(this, new EventArgs());
            }
        }

        private bool isPrime(int number)
        {
            for (int i = 2; i < number-1; i++)
            {
                if ((number % i) == 0) return false;
            }
            return true;
        }
    }

}