阶级使用陷阱打破Liskov替代原则

Question

In a project I worked recently, noticed that some methods that were accepting a class that belongs to a hierarchy , had code similar to following:

public void Process(Animal animal) {
    if(animal.GetType() == typeof(Dog)) {
        Console.WriteLine("We have a dog");
    }
    else {
        Console.WriteLine("not a dog");
    }
}

Well, that struck me as a blatant violation of LSP , because now if you use a subclass of a dog , either in production code, unit testing mock, or as a part of dependency injection interceptor this code will not work the same way . I believe this particular scenario can be fixed easily, by changing condition to:

  if (animal is Dog)

That got me thinking though:

Are there any other pitfalls to look for that can break LSP in client code?

UPDATE

Just to clarify, I am looking for possible pitfalls in code that uses the class in a hierarchy. I am aware and I am not looking for problems with badly contstructed hierarchies - (eg rectangle - square problem). I am trying to find out what to look for, to make sure that the code will support dynamic mocks, interceptors, decorated classes (eg LoggingDog) the same way as it would handle original class.

So far after going through the answers and links I can see that the only pitfall would be to use type of the class directly - that is use GetType() method directly or through some other technology. Despite some comments here is and as operators, and even casting to base type will not break LSP in this case, as sub-classes will evaluate the same way the original class would.

Answer 1

If we ignore for the moment that there are indeed cases where you may need to cast to a specific type, in most cases the problem can be solved by changing the design:

public class Animal
{
    public virtual void Process()
    {
        Console.WriteLine("Animal (not a dog)");
    }
}

public class Dog : Animal
{
    public override void Process()
    {
        Console.WriteLine("We have a dog");
    }
}

By using this design, we avoid the need to cast in the code processing the animals:

var animal = ...; // maybe a Dog, maybe not
animal.Process();