Is it ok to use the same interface definition but provide different behaviour?

Question

the summary of my question is "I have 2 classes which share a common interface, however the two classes have different behaviour with regard to their properties. One interface throws an exception if the parameter values are invalid the other updates its internal state to handle the invalid values. Is it okay for the two classes to share the same interface or should two interfaces be defined to indicate to the developer that the two have different behaviours?"

I'll try to clarify with some code. I have the interface defind below.

public interface IStationDictionary
{
    bool this[string stationId] { get; set; }
}

And a class that implements the interface, this class is used to set the output port on a digital IO board.

public class DigitalStationAdapter : IStationDictionary
{
    public bool this[string stationId]
    {
        get { return ports[stationId].Value; }
        set { ports[stationId].Value = value; }
    }

    public void AddDigitalStation(string stationId, DigitalIoPort port)
    {
        ports.Add(stationId, port);
    }

    private IDictionary<string, DigitalIoPort> ports = new Dictionary<string, DigitalIoPort>();
}

I also have a classes that records which stations have had values changed so that those changes can be propegated to the digital IO.

public class StationTransitions : IStationDictionary
{
    public bool this[string stationId]
    {
        get
        {
            bool result = false;

            if(changes.ContainsKey(stationId))
                result = changes[stationId];

            return result;
        }
        set
        {
            if(!changes.ContainsKey(stationId))
                changes.Add(stationId, value);
            else
                changes[stationId] = value;
        }
    }

    public IDictionary GetChanges()
    {
        IDictionary<string, bool> result = changes;

        changes = new Dictionary<string, bool>

        return result;
    }

    private IDictionary<string, bool> changes = new Dictionary<string, bool>();
}

So, while both classes implement the same interface DigitalStationAdapter will throw a KeyNotFoundException if you try to access the indexer with a stationId not in the dictionary. Whereas, StationTransitions will succeed, i.e. different behaviour. Is this okay, I thought that interfaces are used to define behaviour as well as structure?

Keith.

dsimcha dsimcha · Accepted Answer · 2009-03-02T23:31:09

This is fine. Think of it this way: The key to determining whether any inheritance relationship is "ok" is the Liskov substitution principle. This requires that code expecting a base class should be able to use a derived class without knowing it. This implies that preconditions for a method cannot be strengthened in a derived class but can be weakened. Interfaces are just a special case of base classes.

In your case, the interface's implicit precondition would be that input be valid as you define it. All code that expects the base class should assume this precondition and avoid passing invalid data or be prepared to deal with the exception. In the concrete class that updates its internal state to deal with the invalid input, you are weakening a precondition. This is acceptable as far as the Liskov substitution principle goes.

On the other hand, if you were to make the class that automatically deals with the invalid input w/o throwing a base class, and make the class that throws a descendant, this would be bad, because code expecting the base class could not use the derived class as if it were the base class. It would have to know about the derived class to deal with the possible exception.

Is it ok to use the same interface definition but provide different behaviour?

1 Answers