Why does Dumbledore make Snape DADA teacher?

[Cyril A Fernandes]

No: HPFGUIDX 158585

Cyril...

<major snip>

Steve <bboyminn at ...> wrote:
> As to your parting comment, I definitely agree. Snape was
> the best DADA teacher Harry ever had, but Harry resisted
> that knowledge because it came from Snape. Snape classes
> dealt with things that are critical for Harry to know. He
> presented alternative ways to deal with Dementors, which
> Harry rejected. He taught non-vebal magic which is
> criticial to dueling, but which Harry was terrible at.

<snip>

Hi Steve - can you refresh me as to canon where Snape shows Harry
(or the DADA classes) alternative way to deal with Dementors? I am
not able to locate the same.

I do remember Snape setting them an essay on Dementors - but I do
not recollect him presenting any alternate ways to deal with
Dementors...

Thanks in advance,
Cyril.