[Phenix]

Common law rule.

If the state follows the common law rule - which imposes liability on a dog owner who knew a dog was dangerous - technically, the fact that the injured person was trespassing doesn't matter. So if the common law rule were applied strictly, if you know your dog is dangerous, and it bites a burglar who breaks into your house, you're liable.
In practice, however, courts and juries are reluctant to hold a dog owner liable to a trespasser. Many courts have softened the rule to avoid unjust results. Some have modified the rule to say that a dog owner, even one who knows a dog is dangerous, isn't liable if the dog hurts a trespasser.43 Some say that the common law rule doesn't apply to trespassers if the dog is a guard dog.44
Another way courts get around an unfair result is by allowing the dog owner to charge that the victim, by trespassing, either was partly to blame (see "Was the Injured Person Careless?" below) or knowingly took the risk of injury (see discussion above). If the dog owner can prove either of those circumstances, liability may be reduced or eliminated altogether.

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A Dog Owner's Legal Defenses (美国的,不知加拿大有用没?)

So far, we've talked about the legal rights and options of someone who's injured by a dog. But the dog owner is entitled to fight back with another set of legal theories.

The owner may be able to avoid liability if the injured person:

provoked the injury from the dog
knowingly took the risk of being injured by the dog ("内有恶犬"的sign有用)
was trespassing
was breaking the law, or
was unreasonably careless, and that carelessness contributed to the injury.

This section discusses each of these defenses. It's important to keep in mind three things about them:

Not all defenses can be used in all states.
Sometimes, the defenses that can be used depend on what the dog owner is being sued for. For example, some defenses may be available if the dog owner's liability is based on a common law theory, but not under a dog-bite statute.
Some dog-bite statutes make the victim prove that he or she wasn't at fault - the dog owner doesn't have to prove the victim was at fault. (See Dog-Bite Statutes, above.)