Throwing exceptions in switch statements when no specified case can be handled

Question

Let's say we have a function that changes a password for a user in a system in an MVC app.:

public JsonResult ChangePassword
    (string username, string currentPassword, string newPassword)
{
    switch (this.membershipService.ValidateLogin(username, currentPassword))
    {
        case UserValidationResult.BasUsername:
        case UserValidationResult.BadPassword:
            // abort: return JsonResult with localized error message        
            // for invalid username/pass combo.
        case UserValidationResult.TrialExpired
            // abort: return JsonResult with localized error message
            // that user cannot login because their trial period has expired
        case UserValidationResult.Success:
            break;
    }

    // NOW change password now that user is validated
}

membershipService.ValidateLogin() returns a UserValidationResult enum that is defined as:

enum UserValidationResult
{
    BadUsername,
    BadPassword,
    TrialExpired,
    Success
}

Being a defensive programmer, I would change the above ChangePassword() method to throw an exception if there's an unrecognized UserValidationResult value back from ValidateLogin():

public JsonResult ChangePassword
    (string username, string currentPassword, string newPassword)
{
    switch (this.membershipService.ValidateLogin(username, currentPassword))
    {
        case UserValidationResult.BasUsername:
        case UserValidationResult.BadPassword:
            // abort: return JsonResult with localized error message        
            // for invalid username/pass combo.
        case UserValidationResult.TrialExpired
            // abort: return JsonResult with localized error message
            // that user cannot login because their trial period has expired
        case UserValidationResult.Success:
            break;
        default:
            throw new NotImplementedException
                ("Unrecognized UserValidationResult value.");
            // or NotSupportedException()
            break;
    }

    // Change password now that user is validated
}

I always considered a pattern like the last snippet above a best practice. For example, what if one developer gets a requirement that now when users try to login, if for this-or-that business reason, they're suppose to contact the business first? So UserValidationResult's definition is updated to be:

enum UserValidationResult
{
    BadUsername,
    BadPassword,
    TrialExpired,
    ContactUs,
    Success
}

The developer changes the body of the ValidateLogin() method to return the new enum value (UserValidationResult.ContactUs) when applicable, but forgets to update ChangePassword(). Without the exception in the switch, the user is still allowed to change their password when their login attempt shouldn't even be validated in the first place!

Is it just me, or is this default: throw new Exception() a good idea? I saw it some years ago and always (after groking it) assume it to be a best practice.

Answer 1

I always throw an exception in this case. Consider using InvalidEnumArgumentException, which gives richer information in this situation.

Answer 2

With what you have it is fine although the break statement after it will never be hit because execution of that thread will cease when an exception is thrown and unhandled.

Answer 3

I always like to do exactly what you have, although I usually throw an ArgumentException if it's an argument that was passed in, but I kind of like NotImplementedException better since the error is likely that a new case statement should be added rather than the caller should change the argument to a supported one.

Answer 4

I've used this practice before for specific instances when the enumeration lives "far" from it's use, but in cases where the enumeration is really close and dedicated to specific feature it seems like a little bit much.

In all likelihood, I suspect an debug assertion failure is probably more appropriate.

http://msdn.microsoft.com/en-us/library/6z60kt1f.aspx

Note the second code sample...