Actually, Mandarin is the only one with a standardized written form. Hong Kong people (Cantonese speakers) write formally in Mandarin and pronounce the characters with Cantonese, but they way Cantonese is spoken has different grammar and lexicon. Written Cantonese is non-standardized and often requires new characters that are not found in Mandarin. I don't think there are many people writing Shanghainese because the language is actively discouraged by the government.