when working with multiple t threads, sometimes we need to know when all threads have completed. while there are multiple ways to achieve this. Here is one way I found to be easy.
The key class used to achieve the task is WaitHandle  . lets start with a console app in C#

public static Random random = new Random();

        static void Main(string[] args)
        {
          
            //Initialize the WaitHadlers
            List<WaitHandle> waitHandles = new List<WaitHandle>();

            var manualResetEvent = new ManualResetEvent(false);
            waitHandles.Add(manualResetEvent);
            var taskOne = Task.Factory.StartNew(() => new Thread(JobOne).Start(manualResetEvent));
            taskOne.Wait();

            manualResetEvent = new ManualResetEvent(false);
            waitHandles.Add(manualResetEvent);
            var taskTwo = Task.Factory.StartNew(() =>   JobTwo(manualResetEvent,"A") );
            taskTwo.Wait();

            manualResetEvent = new ManualResetEvent(false);
            waitHandles.Add(manualResetEvent);
            var taskThree = Task.Factory.StartNew(() => new Thread(JobThree).Start(manualResetEvent));      
            taskThree.Wait();

            var waitTask = Task.Factory.StartNew(() => WaitHandle.WaitAll(waitHandles.ToArray()));
            waitTask.Wait();
            Console.WriteLine("-----------------All jobs completed at {0}----------------", DateTime.Now);
            Console.ReadLine();
        }

Here is the methods JobOne, Jobtwo and JobThree

private static void JobOne(Object obj)
        {
            ManualResetEvent manualResetEvent = (ManualResetEvent)obj;

            // simulate a long runningt task
            Console.WriteLine("Job One started at {0} ",DateTime.Now);
            System.Threading.Thread.Sleep(random.Next(5000, 20000)); // random 5 to 20 second delay
            Console.WriteLine("Job One Completed at {0} ", DateTime.Now);

            manualResetEvent.Set();
        }

        private static void JobTwo(Object obj,string a)
        {
            ManualResetEvent manualResetEvent = (ManualResetEvent)obj;

            // simulate some work:
            Console.WriteLine("Job Two started at {0} ", DateTime.Now);
            System.Threading.Thread.Sleep(random.Next(5000, 20000));
            Console.WriteLine("Job Two Completed at {0} ", DateTime.Now);

            manualResetEvent.Set();
        }

        private static void JobThree(Object obj)
        {
            ManualResetEvent manualResetEvent = (ManualResetEvent)obj;

            // simulate some work:
            Console.WriteLine("Job Three started at {0} ", DateTime.Now);
            System.Threading.Thread.Sleep(random.Next(5000, 20000));
            Console.WriteLine("Job Three Completed at {0} ", DateTime.Now);

            manualResetEvent.Set();
        }

You can achieve the same results using the Tasks namespace
See http://www.sammyageil.com/post/2011/06/14/Execute-multiple-Tasks-in-Net-40.aspx