How to solve the "Growing If Statement" problem?

Question

I've been doing some reading about design patterns and wanted some perspective. Consider the following:

Dim objGruntWorker as IGruntWorker

if SomeCriteria then
   objGruntWorker = new GoFor()
else if SomeOtherCriteria then
   objGruntWorker = new Newb()
else if SomeCriteriaAndTheKitchenSink then
   objGruntWorker = new CubeRat()
end if

objGruntWorker.GetBreakfast()
system.threading.thread.sleep(GetMilliSecondsFromHours(4))
objGruntWorker.GetLunch()

The above code grows each time a new Criteria arises. I've seen code like this all over the place and in ignorance wrote some of it myself. How should this be solved? Does this kind of anti-pattern have a more "formal" name? Thanks for your help!

Edit: Another consideration is I want to avoid having to recompile the existing implementations of IGruntWorker simply to add a new implementation.

Answer 1

That sort of logic is often encapsulated using the Factory method pattern. (See the ImageReaderFactory example under Encapsulation.)

Answer 2

The type of pattern that would suit the above solution would be the Factory Pattern. You have a situation where you don't need to know the concrete type of object you require, it just has to implement IGruntWorker. So you create a factory which takes in a criteria and based on that criteria you would return the specific IGruntWorker object. It is usually a good idea to map the criteria to some identifier i.e. an enumeration or constant for readability e.g.

public enum WorkerType
{
    Newbie,
    Average,
    Expert
}

public class WorkerFactory
{
    public static IGruntWorker GetWorker(WorkerType type)
    {
        switch (type)
        {
            case WorkerType.Newbie:
                 return new NewbieWorker();
            case WorkerType.Average:
                 return new AverageWorker();
            case WorkerType.Expert:
                 return new ExpertWorker();
        }
    }
}

So in your case you could have a small helper method that works out the correct type of Worker required based on the criteria. This could even be wrapped up in a read-only property which you just pass into the factory.