Is the usage dash "--" okay in formal writing?

[NewHope]

Context:
On the way, he spoke to us in a perfect native Tianjinese accent. We mentioned Mr.Chern, and he reacted at once --

"Chen Shengshen? A great mathematician, wonderful! Every Tianjinese who has ears knows him! What a talent he is!"

In "at once -- ", I used "--", not "-". Is it okay?

[Mister Micawber]

Fancy meeting you here, MH--

The m-dash is frowned upon in formal writing (essays, theses, business letters, etc.) but you are, I believe, translating fiction or a popular narrative, in which the m-dash is perfectly acceptable-- indeed is often preferable-- to the colon for dramatic effect.

The n-dash is not used in this situation; it is for use between numbers (69-70), etc. Remember not to leave a space at either end of them-- although I do.