writing in cantonese or in mandarin is exactly the same?

the characters used in Cantonese are Traditional (composed of many elements)...
the characters used in Chinese are Simplified (each characters consists of fewer elements)...

for example TV set in Chinese 电视机 and in Cantonese 電視機
horse in Chinese 马 and in Cantonese 馬
questioning particle in Chinese 吗 and in Cantonese 嗎

Chinese pronunciation is represented by pinyin
but
Cantonese pronunciation is represented by Jyutping

as for horse: Chinese ma3, Cantonese maa5
but,
another example,
a word "to ask"...
Chinese: 问 wen4
Cantonese : 問 man6

here is a good dictionary:
http://www.mdbg.net/chindict/chindict.php?page=chardict

Cantonese and Mandarin are written in the same way, the only thing that's different is the pronunciation.

For instance, 緊密 (jin mi), which means "intimately close" in Mandarin is pronounced as "gan mat" instead, but in writing, it is still the same 緊密.

Most people in Cantonese-speaking countries such as Hong Kong and Macau use traditional Chinese instead of simplified Chinese, but it doesn't make any difference. Cantonese and Mandarin are drastically different only in terms of pronunciation.

Cantonese is a dialect, so if someone's writing in Cantonese, they are basically writing out the sounds of the words, in traditional Chinese characters.

Using the examples above:

1. Television
a) Mandarin - 电视机
b) Cantonese - 電視

2. Watch television
a) Mandarin - 看电视
b) Cantonese - 睇電視

It's easier for Cantonese people to read writings in Mandarin than the other way around. Written Mandarin is basically proper Chinese in simplified characters. On the other hand, many of the Cantonese words refer to Cantonese sounds and these sounds are not found in the Mandarin speaking world. For example - "點解" is "why" in Cantonese, but in Mandarin, "why" is "为甚么" (which is also the proper way to write "why" in Chinese).

Ask a Chinese scholar.