Mandatory voting would be dumb.

[Darkwind]

How do illegals vote?
You have to be in the voter registry, and you can't do that unless you are a citizen.
If a person forges a voter registration card or something, that vote will get rejected later.

In many districts, they don't even ask for an ID. They just hand you a ballot.

 

[DGS49]

No, no, no, no, no. The people at the polls have a list of the registered voters in the district/ward/whatever. If you are not a registered voter (or claim to be), you will not be given a ballot. If you take the time to identify a registered voter who DOES NOT VOTE, and hope to impersonate that voter at the polls you are risking being identified and charged with a crime.

The only people who are actually in a position to implement such a nefarious plan are people who work at the polls.

Not that any of them would do such a thing.