WaitHandle[] handles = new WaitHandle[]{
    new AutoResetEvent(false), 
    new AutoResetEvent(false)
};
int times = (int)N/2;
int i;
for(i = 0; i < times; i++){
    ThreadPool.QueueUserWorkItem(new WaitCallback(Tasks[i*2]), handles[0]);   
    ThreadPool.QueueUserWorkItem(new WaitCallback(Tasks[i*2+1]), handles[1]);
    WaitHandle.WaitAll(handles);
}
if(i*2 < N){
    ThreadPool.QueueUserWorkItem(new WaitCallback(Tasks[i*2 + 1]), handles[0]);
    WaitHandle.WaitAny(handles);
}

class CountDownLatch {
    private object lockobj;
    private int counts;

    public CountDownLatch(int counts){
        this.counts = counts;
    }
    public void Await(){
        lock(lockobj){
            while(counts > 0){
                Monitor.Wait(lockobj);
            }
        }
    }
    public void CountDown(){
        lock(lockobj){
            counts--;
            Monitor.PulseAll(lockobj);
        }
    }
}

CountDownLatch cdl = new CountDownLatch(N);
for(int i = 0; i < N; i++){
     ThreadPool.QueueUserWorkItem(new WaitCallback(Tasks[i]), cdl);
}
cdl.Awati();