Kijana Woodard

I inherited some code at work that made use of enums. I happily continued the pattern in order to get the job done considering the tight deadline. My spidey sense kept tingling telling me something was wrong, but I couldn't quite put my finger on it.

The code I started with was pretty standard stuff.

There was an enum:

enum CarType
{
    Slow,
    Fast,
    Lightning
}

There was a class that used the enum:

class Car
{
    public Guid Id { get; private set; }
    public string Name { get; set; }
    public CarType CarType { get; set; }

    public Car() { Id = Guid.NewGuid(); }
}        

There was some List creation:

var cars = new List<Car>()
{
    new Car() { Name = "Yugo", CarType = CarType.Slow },
    new Car() { Name = "M3", CarType = CarType.Fast },
    new Car() { Name = "Tesla Roadster", CarType = CarType.Lightning }
};
        

And then there was branching logic on the enum. This is where the trouble began:

foreach (var car in cars)
{
    switch (car.CarType)
    {
        case CarType.Slow:
            DoSlowCarStuff();           
            break;
        case CarType.Fast:
            DoFastCarStuff();
            break;
        case CarType.Lightning:
            DoLightningCarStuff();
            break;
        default:
            break;
    }
}        

This code was smelly and I was adding more of it. I really didn't understand what I didn't like, but I wanted to do something different.

I decided to use extension methods.

public static class CarTypeExtensionMethods
{
    public static void DoCarStuff(this CarType type)
    {
        switch (type)
        {
            case CarType.Slow:
                DoSlowCarStuff();
                break;
            case CarType.Fast:
                DoFastCarStuff();
                break;
            case CarType.Lightning:
                DoLightningCarStuff();
                break;
            default:
                break;
        }
    }

    static void DoSlowCarStuff() { }
    static void DoFastCarStuff() { }
    static void DoLightningCarStuff() { }
}        

So now my consuming code looked like this:

foreach (var car in cars)
    car.CarType.DoCarStuff(); 
       

Ahhhh. Now that's bliss. All the car type junk was packaged together and the calling code is dead simple.

But something still felt...wrong. The big "switches" were all gone, but I still had some "if (carType ==" statements lying around. I could put those in the extension methods, but that wasn't really the root issue.

I went to the Big G and typed something like "c# alternatives to enums". Somewhere along the line I stumbled on this post about comparing c# enums to java enums.

At first, I thought, this looks like a ton more code to write for little gain. But it felt like the right direction. I decided to just write it and see what happened.

class CarType
{
    public static readonly CarType Slow = new CarType()
    {
        _display = "Slow",
        _dostuff = () =>
        {
            //do slow car stuff
        }
    };

    public static readonly CarType Fast = new CarType()
    {
        _display = "Fast",
        _dostuff = () =>
        {
            //do fast car stuff
        }
    };

    public static readonly CarType Lightning = new CarType()
    {
        _display = "Lightning",
        _dostuff = () =>
        {
            //do lightning car stuff
        }
    };

    public override string ToString()
    {
        return _display;
    }

    public void DoCarStuff()
    {
       _dostuff();
    }
    private CarType() { }
    private string _display;
    private Action _dostuff;
}        

And suddenly the real problem with the original code was obvious. Switch/If blocks were littered everywhere through the program. If you added a new CarType, you'd have to hunt through the entire application updating the switch/if logic.

The extension method class was better in that the code was all in one class, but you still had to go through and update it all.

Now, when you create a new "enum" type, all the logic is done right there. Even typing up this blog post I smiled when I didn't have to change my Car class or the consuming code that called DoCarStuff(). I can add CarTypes at will, knowing I don't have to change any other code.

Enums are still useful when all they do is identity. As soon as you start branching on enums, switch to a class. You'll thank me later.

So by now, you might be thinking, congratulations, you've discovered the strategy pattern. I get that. However, I find it useful to think about solving concrete problems like getting rid of branching code on enums by using proper classes. It's the same thing, but if I just said, "use the strategy pattern", a lot of people, myself included, would leave the blog post less informed.

Finally, I know some of you might think that this code is terrible:

car.CarType.DoCarStuff();        

I realize that Car should probably define DoCarStuff and not expose it's CarType, but this was the first example I could think of and I figured I'd write the post instead of trying to think of the perfect example.

Thoughts?