Having lived in China for 5 years, I don't think they actually like North Korea that much. There is a sense that they are like family given past alliance and similarities, cult of personality, etc. But while china now advances and moves forward, NK is like their little embarrassing little brother that dropped out of high school and they reluctantly have to try to keep them out of trouble.
The feel of China is that the top leadership are hell bent on making themselves the dominant world power of the 21st century. outsourcing, currency manipulation, and American debt are all parts of this plan. China would get pissed at an invasion of NK, not because they like them, but because it would be an invasion of "their" territory and sphere of influence.