***方法是使方法async生效，因为编译器错误指示:

The best approach is to make the method async, as the compiler error indicates:

public async Task<Stream> CreateSerialPortStreamAsync(string serialPortName)

这也需要更改界面:

Task<Stream> CreateSerialPortStreamAsync(string serialPortName);

是的，此方法的所有调用方都需要更新.

And yes, all callers of this method will need to be updated.