What should be the last entry in a switch/case statement?

Question

When writing a switch statement that only ever has to deal with a known set of values (imagine an implicit enumeration), I find myself wondering what should be the last entry in the construct. I'm always considering the same 3 approaches, but I'd really like to just stick with the one that makes the most sense and never think about this again. The alternatives are described below. For the sake of discussion, let's assume there are exactly 4 options for the "switching variable".

• Approach #1: 4 cases, no default case.
• Approach #2: 3 cases, default acts as the last case (+ comment explaining this).
• Approach #3: 4 cases, default case with a "ShouldNeverHappenException".

My thoughts on this are as follows:

• One the one hand, since default is effectively a case that cannot be reached, it seems pointless to clutter the code with it.
• On the other hand, it's bad not to handle a default case out of of future-proofing considerations, that is, if someday another option becomes available (due to e.g. API changes outside of my control), the code might react to it incorrectly if the default case is used for one of the expected options.
• If the last case handles an error scenario, it might not be so bad if it's the default behavior (assuming I have no fine-grained error handling).

Based on the above reasoning I tend to prefer the 3^rd approach, but I am no software engineer, and perhaps I'm missing something.

What are the best practices in this case?

Answer 1

imagine an implicit enumeration

I think this is the key point. Implicit means not an actual enum type, but, say, numbers with special meaning.

const int A = 1;
const int B = 2;
const int C = 3;

And a method that is using the switch statement:

public void DoSomething(int type)
{
    switch (type)
    {
        case A:
            // Stuff
            break;
        case B:
            // Stuff
            break;
        case C:
            // Stuff
            break;
        default:
            // Do I even needs this?
            break;
    }
}

The "implicit" enumeration in this example is a 32 bit integer. The entire range of values for the 32 bit integer are your possible cases. So what if I call:

DoSomething(355);

Or:

DoSomething(-2);

You might say "I'm always using the constants A, B and C in my program when calling this method, so it's impossible to pass -1"

But the compiler allows it, therefore a code change in the future in how DoSomething is called can pass an illegal value.

If you pass an illegal value, is there an intelligent default you can fall back on? If so, that's your default. If not, throw an exception.

I would argue for a default even for an explicit enumeration:

enum Foo
{
    A = 1,
    B = 2,
    C = 3
}

public void DoSomething(Foo type)
{
    switch (type)
    {
        case A:
            // Stuff
            break;
        case B:
            // Stuff
            break;
        case C:
            // Stuff
            break;
        default:
            throw new NotImplementedException("Type " + type + " is not yet implemented");
            break;
    }
}

The default clause throwing an exception to the fact a value is not implemented is a good debugging tool for the future where you add an item to the enum, and then you forget to change the application behavior based on that.

Will the exception happen in production? Most likely not. It will probably happen during development or testing and be fixed.

But that's the benefit of having a default clause and either doing something by default or blowing up.

There are some cases where doing nothing is OK. I honestly would still put in a default clause with a comment:

public void DoSomething(int type)
{
    switch (type)
    {
        case A:
            ...
            break;
        case B:
            ...
            break;
        case C:
            ...
            break;
        default:
            // Do nothing intentionally
            break;
    }
}

To me this falls under the same guidelines as exception handling. If you catch the exception, do something with it. If you don't want to do anything with it, catch the exception and explain why nothing is done:

try
{
    ...
}
catch(Exception)
{
    // Do nothing, because we are trying to do something and we
    // really don't care if it happens.
}

Answer 2

I remember early in my career writing a series of if else if statements which I thought covered all cases.

At the end I added:

else 
{
    Throw new Exception("THE IMPOSSIBLE HAPPENED!!");
}

Of course some months later my boss approached me with a worried look on his face, "Ewan, what does 'The impossible happened' mean?!".

It means you have forgotten about nulls, that's what it means

Answer 3

If you're really dealing with an enumerated type, for which there are a fixed and relatively small set of values, I see no real reason to have a default case:

• Using it as a form of "pre-debugging" to catch your potential errors for you seems to have little value. In fact, it's totally useless if you've written your switch right the first time. You could just take the extra time you would have spent adding the default case to give your switch another once-over for errors.

• Using it as a form of future-proofing to catch potential errors from later code modifications seems to have little value as well. Say someone adds more enumerations later. If they are diligent, they will correctly add their new case to the switch and the default would again have served no purpose. And if they are not diligent in paying attention to the existing switch code, your future-proofing could just as easily cause a problem instead of solving it. Maybe they wanted the switch to do nothing at all with their new enumeration (i.e. no default case), but they failed to see you have put a default in. Your future-proofing would have then made a little extra work for them.

Ultimately, it will depend on your situation and how well "known" your set of values is. If the set of values is large, or you don't trust in your capabilities to completely "know" what they are or could be, maybe a catch-all default is worth the added peace of mind.

Answer 4

switch country
US: minimum_drinking_age = 21
Spain: minimum_drinking_age = 16
default: minimum_drinking_age = 18

I just pulled these numbers out of my nose. In there, I assume in US the legal age of drinking is 21+, in spain its 16+ and in every other country in the world its 18+.

This is just a random scenario. There are just 2 cases where a value must be set specifically, in every other case, it will default to one value. Why would you throw an exception? Makes no sense. The default is used for defaults. Misusing it for other purposes is besides the point!

According to this it seems like I was not quite off (18 is default)!