Is type checking ever OK?

Question

Is type checking considered bad practice even if you are checking against an interface? I understand that you should always program to an interface and not an implementation - is this what it means?

For example, in PHP, is the following OK?

if($class instanceof AnInterface) {
   // Do some code
}

Or is there a better way of altering the behaviour of code based on a class type?

Edit: Just to be clear I am talking about checking whether a class implements an interface not just that it is an instance of a certain class.

Answer 1

Run-time type checking is often necessary in situations where an interface provides all the methods necessary to do something, but does not provide enough to do it well. A prime example of such a situation is determining the number of items in an enumerable sequence. It's possible to make such a determination by enumerating through the sequence, but many enumerable objects "know" how many items they contain. If an object knows how many items it contains, it will likely be more efficient to ask it than to enumerate through the collection and count the items individually.

Arguably, IEnumerable should have provided some methods to ask what it knows about the number of items it contains [recognizing the possibility that the object may know that the number is unbounded, or that it's at most 4,591 (but could be a lot less), etc.], but it doesn't. What might be ideal would be if a new version of IEnumerable interface could be produced that included default implementations for any "new" methods it adds, and if such interface could be considered to be implemented by any implementations of the present version. Unfortunately, because no such feature exists, the only way to get the count of an enumerable collection without enumerating it is to check whether it implements any known collection interfaces that include a Count member.

Answer 2

As long as you follow the LSP, I don't see a problem. Your code must work with any implementation of the interface. It's not a problem that certain implementations cause you to follow different code paths, as long as you can correctly work with any implementation of the interface.

If your code doesn't work with all implementations of the interface, then you shouldn't use the interface in the first place.

Answer 3

In my practice any checking for type (as well as type casting) has always indicated that something is wrong with the code or with the language.

So I try to avoid it whenever possible.

Answer 4

If you can avoid type checking you should; however, one scenario where I found it handy, was we had a web service which took a message but the contents of the message could change. We had to persist the message back into a db, in order to get the right component to break the message down to its proper tables we used type checking in a sense.

What I find more common and flexible then if ($class instanceof SomeOtherType) is to define an IProcessing strategy for example and then using factory based on the type $class create the correct class.

So in c# roughly this:

void Process(Message msg)
{
    IProcessor processor=ProcessignFactory.GetProcessor(msg.GetType()); 
    processor.Process(msg); 
}

However sometimes doing this can be overkill if your only dealing with one variation that won't change implement it using a type check, and when / if you find you were wrong and it requires more checks then refactor it into a more robust solution.