C# 多线程 lock ，有两个线程 要求只能一个一个执行（线程同步）写法有什么错误

Question

private static object lockobj = new object(); 
private void button3_Click(object sender, EventArgs e)
        {
            lock (lockobj)
            {
                Thread thr = new Thread(changetext);
                thr.Name = "t1";
                thr.IsBackground = true;
                thr.Start();
            }
            Thread thr2 = new Thread(changetext);
            thr2.Name = "t2";
            thr2.IsBackground = true;
            thr2.Start();
        }

Answer 1

 
                Thread thr = new Thread(changetext);
                thr.Name = "t1";
                thr.IsBackground = true;
                thr.Start();
             thr.Join();            Thread thr2 = new Thread(changetext);
            thr2.Name = "t2";
            thr2.IsBackground = true;
            thr2.Start();

Answer 2

将lock (lockobj)放到changetext里

Answer 3

thr.Join();
这个是什么意思 处理线程同步的另一种方法吗？