Like many myths, the myth that the many forms of Chinese are identical in writing is false but has a kernel of truth. That kernel of truth is that someone who can read one form of Chinese has a fairly easy time learning to read another. For one thing, the written form erases the numerous differences in pronunciation. For another, until fairly recently there was a more-or-less standard written form not identical to any spoken dialect. Just as Europeans who spoke various languages could communicate in Latin, so until recently all literate Chinese people could communicate in the somewhat artificial written standard. With the shift over the last century to a written language that is closer to the colloquial, the differences among dialects have increased. Even so, people whose native language is something other than Mandarin often write in what is more-or-less written Mandarin, not the written version of their own dialect. Indeed, some dialects don’t really have a written form, and even those that do typically do not have characters for all of the words that are not cognate to the standard Chinese word.
Language Log » Identifying written Cantonese
