I'm not an expert on Canadian law, but legally speaking the answer seems to be yes, unless you hold an exemption or travel to Canada overland via the United States.

1) The Government of Canada Help Center specifies that:

Please Note: Canadian citizens (including dual citizens) cannot apply for an eTA.

2) The Canadian eTA application website asks you the following question:

Indicate if you are a citizen of a country/territory other than the one on your passport.

3) Specifying 'Canada' as a second country of citizenship will (most likely) result in a refused eTA application. Failing to include that information will result in the crime of Document Fraud (Misrepresentation):

It is a serious crime to lie, or to send false information or documents, when you deal with Citizenship and Immigration Canada (CIC). This is fraud. It is called “misrepresentation.”

A more legalese definition is provided in Immigration and Refugee Protection Act, SC 2001, c 27, s 127:

Misrepresentation

127. No person shall knowingly

(a) directly or indirectly misrepresent or withhold material facts relating to a relevant matter that induces or could induce an error in the administration of this Act;

4) The official penalty for the crime of Misrepresentation is defined in the Immigration and Refugee Protection Act, SC 2001, c 27, s 128:

Penalties

128. A person who contravenes a provision of section 126 or 127 is guilty of an offence and liable

(a) on conviction on indictment, to a fine of not more than $100,000 or to imprisonment for a term of not more than five years, or to both; or

(b) on summary conviction, to a fine of not more than $50,000 or to imprisonment for a term of not more than two years, or to both.

There are also the possibly penalties of deportation and/or entry bans, however they obviously don't apply to Canadian citizens. The only possible options for Canadian dual citizens to travel to Canada are therefore:

1. Fly using their Canadian passports
2. Enter Canada over land or water via the US, where an eTA is not needed
3. Apply for a special exemption from the Canadian authorities, allowing you to enter Canada on a foreign passport.

Update: This page clarifies that starting November 10, 2016, Canadian dual citizens will no longer be able to enter Canada with a foreign visa-exempt passport (other than a US passport). However, until January 31, 2017, people meeting certain conditions may apply for a special authorization that, if approved, will allow the person to board the flight.

Not illegal, but maybe practically impossible, at least by air.

This FAQ question directly address your question: I am a Canadian citizen and a citizen of a visa-exempt country. Can I still fly to Canada with my foreign passport?

It confirms that currently (I guess before eTA is required), you can enter Canada using a foreign passport. But it says that once Canada starts requiring airlines to verify proof that the person can enter Canada (I guess this is after eTA is required?), if you present a foreign passport, you may face delays or be denied boarding. It doesn't say why, but that is presumably because, as it says later, Canadian dual citizens cannot apply for eTA.

Note that eTA is not required for entry by land or sea, so that route is probably still available for a Canadian citizen to enter with a non-US visa-exempt foreign passport.