NikitaClear

using System;
using System.Threading;

namespace ConsoleApplication1
{
    class Program
    {
        public static int[] A;
        public static int[] C;
        public static int CountElements;
        public static int CountThreads;
        public static int Result = 0;
        public static object Locker = new object();

        static void Main(string[] args)
        {
            string input;
            int subArrayLen = 0;
            Console.Write("Arrays count elements: ");
            input = Console.ReadLine();

            if (!int.TryParse(input,out CountElements) || CountElements <= 0)
            {
                CountElements = 10;
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine($"Input error. count elements - {CountElements}");
                Console.ResetColor();
            }
            A = new int[CountElements];
            C = new int[CountElements];
            Console.Write("Threads count: ");
            input = Console.ReadLine();

            if (!int.TryParse(input,out CountThreads) || CountThreads <= 0)
            {
                CountThreads = 1;
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine($"Input error. count threads - {CountThreads}");
                Console.ResetColor();
            }

            if (CountThreads > CountElements)
            {
                CountThreads = CountElements;
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine($"Threads count bigger then count elements. Threads count - {CountThreads}");
                Console.ResetColor();
            }
            subArrayLen = CountElements / CountThreads + 1;
            Console.WriteLine("Fill array A:");
            FillArray(ref A);
            Console.WriteLine("Fill array C:");
            FillArray(ref C);
            Console.WriteLine("A:");
            ShowArray(A);
            Console.WriteLine("\nC:");
            ShowArray(C);
            Thread[] threads = new Thread[CountThreads];

            for (int i = 0; i < CountThreads; i++)
            {
                PartArray partArray = new PartArray
                {
                    StartIndex = i * subArrayLen,
                    EndIndex = (i + 1) * subArrayLen
                };

                if (partArray.EndIndex > CountElements)
                    partArray.EndIndex = CountElements;
                threads[i] = new Thread(() => GetProductNumbers(partArray));
                threads[i].Name = $"Поток№{i + 1}";
                threads[i].Start();
            }
            Console.WriteLine();

            foreach (Thread thread in threads)
            {
                Console.WriteLine($"{thread.Name} - on");
                thread.Join();
                Console.WriteLine($"{thread.Name} - off");
            }
            Console.WriteLine($"Result of product: {Result}");
            Console.ReadLine();
        }

        private static void FillArray(ref int[] A)
        {
            Random random = new Random();

            for (int i = 0; i < A.Length; i++)
            {
                Console.Write($"{i + 1} element:");

                if (!int.TryParse(Console.ReadLine(), out A[i]))
                {
                    A[i] = random.Next(1, 10);
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine($"Input error. element - {A[i]}");
                    Console.ResetColor();
                }
            }
        }

        private static void ShowArray(int[] array)
        {
            foreach (int item in array)
                Console.Write($"{item} ");
        }

        private static void GetProductNumbers(PartArray partArray)
        {
            int start = partArray.StartIndex;
            int end = partArray.EndIndex;

            for (int i = start; i < end; i++)
                lock (Locker)
                    Result += A[i] * C[i];
        }
    }

    class PartArray
    {
        public int StartIndex;
        public int EndIndex;
    }
}