How to use ～に同意する to agree or disagree with a person?


A couple of times I've tried to say "I agree with..." or "he agrees with..."
but I don't know whether I'm using `同意{どうい}する` correctly.

In what ways should `に同意{どうい}している` be used? Also, which particles should be
used? Can only `に` be used before `同意{どうい}する` or are there circumstances where
other particles are used as well, such as `と` and `を`?

In many examples I've found using `同意{どうい}する`, I've seen `（だ）ということに` or just
`（な）ことに` (the `だ`/`な` after na adjectives/nouns and without `だ`/`な` after i
adjectives/verbs it seems.) Is there a specific reason for using one over the
other, e.g. is the longer `（だ）ということに` more polite, and what do these mean?

  
Some example sentences:

  * I agree with his opinion: 

> `彼{かれ}の意見{いけん}に同意{どうい}します。`

  * I agree with him that oranges are tasty: 

> `オレンジはおいしいということ彼{かれ}に同意{どうい}します。`

  * He agrees that apples are tasty: 

> `彼{かれ}はりんごがおいしいことに同意{どうい}します。`

  * Many people agree that brussel sprouts don't taste nice: 

> `芽{め}キャベツはおいしくないということに同意{どうい}する人{ひと}が多{おお}い。`

  
**See also:** [What are the different ways of saying to agree or disagree with
a person?](https://japanese.stackexchange.com/questions/3522/) for other ways
of agreeing/disagreeing with someone.




You can find a lot of examples
[here](http://eow.alc.co.jp/%E5%90%8C%E6%84%8F/UTF-8/?pg=1) (you may want to
check later pages), which will explain a lot.

It seems that we do use "に" for "同意する".

"ということ" literally means "the so-called", which I think is a little more
polite.

Also to @sawa, search in the above site with
"[いという](http://eow.alc.co.jp/%E3%81%84%E3%81%A8%E3%81%84%E3%81%86/UTF-8/)" and
"[いだという](http://eow.alc.co.jp/%E3%81%84%E3%81%A0%E3%81%A8%E3%81%84%E3%81%86/UTF-8/)"
(I know they are not words nor phrases ...) gives interesting results. It
seems that wile i adjectives, "美味しいだということ" can be used.

And I think "オレンジはおいしいということ彼に同意します" is still a little weird, maybe it should
be "オレンジ **が** おいしいだということ **について** 彼に同意します。". A phrase with nothing in the end
doesn't sound natural. (About "は" and "が", I just feel this way, I am not able
to explain it in details.)

BTW, I am not native, so correct me if I am wrong.

