如何从另一个线程锁定方法的一部分？

一种有效的方式是使用互锁的交换机。通过在工作期间将某些标记字段设置为非默认值，其他线程可以检查并退出。例如：

private int hazWorker; // = 0 - put this at the scope you want to protect

然后：

// means: atomically set hazWorker to 1, but only if the old value was 0, and
// tell me what the old value was (and compare that result to 0)
if(Interlocked.CompareExchange(ref hazWorker, 1, 0) != 0) {
    return; // someone else has the conch
}
try {
    // your work here
} finally {
    Interlocked.Exchange(ref hazWorker, 0); // set it back to default   
}