Question

UPDATED I’ve updated the example to better illustrate my problem. I realised it was missing one specific point – namely the fact that the CreateLabel() method always takes a label type so the factory can decide what type of label to create. Thing is, it might need to obtain more or less information depending on what type of label it wants to return.

I have a factory class that returns objects representing labels to be sent to a printer.

The factory class looks like this:

public class LargeLabel : ILabel {     public string TrackingReference { get; private set; }      public LargeLabel(string trackingReference)     {         TrackingReference = trackingReference;     } }  public class SmallLabel : ILabel {     public string TrackingReference { get; private set; }      public SmallLabel(string trackingReference)     {         TrackingReference = trackingReference;     } }  public class LabelFactory {     public ILabel CreateLabel(LabelType labelType, string trackingReference)     {         switch (labelType)         {             case LabelType.Small:                 return new SmallLabel(trackingReference);             case LabelType.Large:                 return new LargeLabel(trackingReference);         }     } } 

Say that I create a new label type, called CustomLabel. I want to return this from the factory, but it needs some additional data:

public class CustomLabel : ILabel {     public string TrackingReference { get; private set; }     public string CustomText { get; private set; }      public CustomLabel(string trackingReference, string customText)     {         TrackingReference = trackingReference;         CustomText = customText;     } } 

This means my factory method has to change:

public class LabelFactory {     public ILabel CreateLabel(LabelType labelType, string trackingReference, string customText)     {         switch (labelType)         {             case LabelType.Small:                 return new SmallLabel(trackingReference);             case LabelType.Large:                 return new LargeLabel(trackingReference);             case LabelType.Custom:                 return new CustomLabel(trackingReference, customText);         }     } } 

I don’t like this because the factory now needs to cater for the lowest common denominator, but at the same time the CustomLabel class needs to get a custom text value. I could provide the additional factory method as an override, but I want to enforce the fact that the CustomLabel needs the value, otherwise it’ll only ever be given empty strings.

What is the correct way to implement this scenario?

Answer 1

Well, how do you want to call the factory method?

Concentrate on how you want to be able to use your API, and the implementation will usually make itself fairly clear. This is made even easier if you write the desired results of your API as unit tests.

An overload may well be the right thing to do here, but it really depends on how you want to use the factory.