进程的输出如何输出到控件的属性？

Question

我写了一下代码，目的是将returnStr的值能够打印到调用它的form的textbox1.text中。需要实时的打印。这怎么做？似乎进程是个难点吧？   
public class processProgram001
    {
        public int sleepTime;
        public Random random = new Random();
        public string returnStr;

        public processProgram001(ref string returnStr)
        {
            sleepTime = random.Next(3000);    //3000毫秒
        }

        public void print()
        {
            Thread current = Thread.CurrentThread;
            string returnStr= current.Name + " going to sleep for " + sleepTime + " milliseconds\r\n";
            Thread.Sleep(sleepTime);
            returnStr += current.Name + " done sleeping";
        }
    }