C#

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

//  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
namespace scratchpad3
{
    class Program
    {
        public static void Main()
        {
            int[] a = { 1, 2, 3, 4, 5 };
            MyThread mt1 = new MyThread("Child #1", a);
            MyThread mt2 = new MyThread("Child #2", a);
            mt1.Thrd.Join();
            mt2.Thrd.Join();
        }
    }

    //>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    class SumArray
    {
        int sum;
        object lockOn = new object(); // a private object to lock on
        public int SumIt(int[] nums)
        {
            lock (lockOn)
            { // lock the entire method
                sum = 0; // reset sum
                for (int i = 0; i < nums.Length; i++)
                {
                    sum += nums[i];
                    Console.WriteLine("Running total for " + Thread.CurrentThread.Name +  " is " + sum);
                    Console.Read();
                    Thread.Sleep(10); // allow task-switch
                }
                return sum;
            }
        }
    }

  //  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

class MyThread {
public Thread Thrd;
int[] a;
int answer;
// Create one SumArray object for all instances of MyThread.
static SumArray sa = new SumArray();
// Construct a new thread.
public MyThread(string name, int[] nums)
{
    a = nums;
    Thrd = new Thread(this.Run); // ****************************
    Thrd.Name = name;
    Thrd.Start(); // start the thread
}
// Begin execution of new thread.
void Run()
{
    Console.WriteLine(Thrd.Name + " starting.");
    answer = sa.SumIt(a);
    Console.WriteLine("Sum for " + Thrd.Name + " is " + answer);
    Console.WriteLine(Thrd.Name + " terminating.");
    Console.Read();
}
}
    //>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

}

 Child #1 starting.
Running total for Child #1 is 1
Child #2 starting.
Running total for Child #1 is 3
Running total for Child #1 is 6
Running total for Child #1 is 10
Running total for Child #1 is 15
Running total for Child #2 is 1
Sum for Child #1 is 15
Child #1 terminating.
Running total for Child #2 is 3
Running total for Child #2 is 6
Running total for Child #2 is 10
Running total for Child #2 is 15
Sum for Child #2 is 15
Child #2 terminating.