When to use an interface instead of an abstract class and vice versa?

I wrote an article about that:

Abstract classes and interfaces

Summarizing:

When we talk about abstract classes we are defining characteristics of an object type; specifying what an object is.

When we talk about an interface and define capabilities that we promise to provide, we are talking about establishing a contract about what the object can do.

Personally, I almost never have the need to write abstract classes.

Most times I see abstract classes being (mis)used, it's because the author of the abstract class is using the "Template method" pattern.

The problem with "Template method" is that it's nearly always somewhat re-entrant - the "derived" class knows about not just the "abstract" method of its base class that it is implementing, but also about the public methods of the base class, even though most times it does not need to call them.

(Overly simplified) example:

abstract class QuickSorter
{
    public void Sort(object[] items)
    {
        // implementation code that somewhere along the way calls:
        bool less = compare(x,y);
        // ... more implementation code
    }
    abstract bool compare(object lhs, object rhs);
}

So here, the author of this class has written a generic algorithm and intends for people to use it by "specializing" it by providing their own "hooks" - in this case, a "compare" method.

So the intended usage is something like this:

class NameSorter : QuickSorter
{
    public bool compare(object lhs, object rhs)
    {
        // etc.
    }
}

The problem with this is that you've unduly coupled together two concepts:

1. A way of comparing two items (what item should go first)
2. A method of sorting items (i.e. quicksort vs merge sort etc.)

In the above code, theoretically, the author of the "compare" method can re-entrantly call back into the superclass "Sort" method... even though in practise they will never want or need to do this.

The price you pay for this unneeded coupling is that it's hard to change the superclass, and in most OO languages, impossible to change it at runtime.

The alternative method is to use the "Strategy" design pattern instead:

interface IComparator
{
    bool compare(object lhs, object rhs);
}

class QuickSorter
{
    private readonly IComparator comparator;
    public QuickSorter(IComparator comparator)
    {
        this.comparator = comparator;
    }

    public void Sort(object[] items)
    {
        // usual code but call comparator.Compare();
    }
}

class NameComparator : IComparator
{
    bool compare(object lhs, object rhs)
    {
        // same code as before;
    }
}

So notice now: All we have are interfaces, and concrete implementations of those interfaces. In practise, you don't really need anything else to do a high level OO design.

To "hide" the fact that we've implemented "sorting of names" by using a "QuickSort" class and a "NameComparator", we might still write a factory method somewhere:

ISorter CreateNameSorter()
{
    return new QuickSorter(new NameComparator());
}

Any time you have an abstract class you can do this... even when there is a natural re-entrant relationship between the base and derived class, it usually pays to make them explicit.

One final thought: All we've done above is "compose" a "NameSorting" function by using a "QuickSort" function and a "NameComparison" function... in a functional programming language, this style of programming becomes even more natural, with less code.

An abstract class can have shared state or functionality. An interface is only a promise to provide the state or functionality. A good abstract class will reduce the amount of code that has to be rewritten because it's functionality or state can be shared. The interface has no defined information to be shared