to be an expert

• 门门在行

  an expert in all areas

• 经济问题他很在行。

  He is very knowledgeable when it comes to economic issues.

• 这种事他很在行。

  He is an expert at this sort of thing.

• 这方面我不大在行。

  I am not very knowledgeable in this area.

• 做生意我可一点也不在行。

  I am a total amateur when it comes to doing business.