foreach (string number in liststrings)
        {
            ThreadStart work = delegate { Method1(number); };
            Thread thr = new Thread(work);
            thr.IsBackground = true;
            thr.Start();
        }

public static Method1(string numb)
{
    Random rnd = new Random();
    int threadC = rnd.Next(1,100000);
    MessageBox.Show(threadC.ToString());
    int i = 1;
    while(i==1)
    {
        //Do Stuff
    }
}