Mandarin is the official language of China. Every chinese, who had school education, can speak it. You can be known everywhere if you speak mandarin.

you would be understood well, it's just sometimes old people like to speak shanghainese, or some people from the coutryside don't speak standard mandarin, you might not understand them well :)

if you want to be understood well by people in Shanghai , U only should learn mandarin well. There are many foreigners in Shanghai.Most of them speak mandarin, the shanghai people can know what they say,and shanghai people will speak mandarin to them to make sure the foreigners can understand.So don't worry!

In fact, Wunese has some dialects too, such as Shanghainese, Suzhounese, Hangzhounese, and so on... If you really want to learn Wunese, you have to settle down in Shanghai and practise your Shanghainese.

Wunese, which is spoken in Shanghai & surrounding places, is harder than Mandarin, because it remains a lot of elements from Classical Chinese. But you'll be understood in Shanghai if you speak Mandarin well. Don't worry.