Which Exception Should I Throw To Signal an Internal Error in my Program?

Question

Which exception should I use when the program reaches a logic state that I "know" won't happen, and if it does, something is terribly bad.

For example:

int SomeFunction(int arg) {
    SomeEnum x = Whatever(arg, somePrivateMember);
    switch (x) {
        case SomeEnum.Value1:
            return SomeFunction1();
        case SomeEnum.Value1:
            return SomeFunction2();
        default:
            throw new WhatTypeToThrow();
    }
}

Clearly, ArgumentException is a long-shot here since the invalid value for x could have come from a bug in Whatever(), or an invalid combination of any arguments and/or the current instance state.

I'm looking for something such as an InvalidProgramStateException, InternalErrorException or similar.

Of course I could define my own, but I wonder if there is a suitable exception in the framework.

Edit: Removed the simple sample code to reduce amount of ArgumentException answers.

Answer 1

What about InvalidOperationException?

Answer 2

I think ArgumentOutOfRangeException is valid here and it's what I use. It's the argument to the switch statement that is not handled as it's out of the range of handled values. I tend to code it like this, where the message tells it like it is:

switch (test)
{
    case SomeEnum.Woo:
        break;
    case SomeEnum.Yay:
        break;
    default:
    {
        string msg = string.Format("Value '{0}' for enum '{1}' is not handled.", 
            test, test.GetType().Name);

        throw new ArgumentOutOfRangeException(msg);
    }
}

Obviously the message is to your own tastes, but the basics are in that one. Adding the value of the enum to the message is useful not only to give detail concerning what known enum member was not handled, but also when there is an invalid enum i.e. the old "(666)SomeEnum" issue.

Value 'OhNoes' for enum 'SomeEnum' is not handled.

vs

Value '666' for enum 'SomeEnum' is not handled.

Answer 3

Here are suggestions that I've been given:

• ArgumentException: something is wrong with the value

• ArgumentNullException: the argument is null while this is not allowed

• ArgumentOutOfRangeException: the argument has a value outside of the valid range

Alternatively, derive your own exception class from ArgumentException.

An input is invalid if it is not valid at any time. While an input is unexpected if it is not valid for the current state of the system (for which InvalidOperationException is a reasonable choice in some situations).

See similar question and answer that I was given.

Answer 4

"program reaches a logic state that I "know" won't happen, and if it does, something is terribly bad."

In this case I would throw an ApplicationException, log what you can and exit the app. If things are that screwed up, you certainly shouldn't try to recover and/or continue.