.NET: Which Exception to Throw When a Required Configuration Setting is Missing?

Question

Here\'s a standard scenario:

if(string.IsNullOrEmpty(Configuration.AppSettings[\"foobar\"]))
   throw new SomeStandardException(\"Application not configured          


        

Answer 1

You could try inheriting off the XML Exception, or just using it.

Answer 2

Personally, I'd use InvalidOperationException, as it's a problem with the object state - not the configuration system. After all, shouldn't you allow these settings to be set by code and not config as well? The important part here is not that there was no line in app.config, but that a required piece of info was not present.

To me, ConfigurationException (and it's replacement, ConfigurationErrorsException - despite the misleading MSDN docs) are for errors in saving, reading, etc. of Configuration.

Answer 3

My general rule would be:

1. If the case of the missing configuration is not very common and I believe I would never want to handle this case differently than other exceptions, I just use the basic "Exception" class with an appropriate message:

   throw new Exception("my message here")

2. If I do want, or think there's a high probability I would want to handle this case in a different manner than most other exceptions, I would roll my own type as people have already suggested here.

Answer 4

You're not limited in your exception-throwing to existing exceptions in the Framework. If you do decide to use existing exceptions, you don't absolutely have to follow the documentation to the letter. The documentation will describe how the framework uses a given exception, but doesn't imply any limitation on how you choose to use/reuse an existing exception.

It's your application- as long as you document it and clearly indicate the exception that will be thrown in the specific case of a missing configuration value, you can use any exception you like. If you do want a very specific indication of a missing value, you might consider writing your own ConfigurationSettingMissing exception:

[Serializable]
public class ConfigurationMissingException : ConfigurationErrorsException
{}

EDIT: Writing your own exception in this case carries the added benefit of guaranteeing that there will never be any confusion regarding where the exception is coming from- the framework, or your application. The framework will never throw your custom exceptions.

UPDATE: I agree with the comments, so I have changed the subclass to ConfigurationErrorsException from Exception. I think it's generally a good idea to subclass custom exceptions from existing Framework exceptions where possible, avoiding the Exception class unless you need an application-specific exception.

Answer 5

An alternative method you could use for your config files would be to use custom configuration sections as opposed to AppSettings. That way you can specify that a property IsRequired and the configuration system will handle this checking for you. If the property is missing it will throw a ConfigurationErrorsException so I suppose that supports the answer that you should use that exception in your case.

Answer 6

I'd suck it up and roll my own... but before you do that, Is it possible to have the system assume a default value for this configuration setting? I generally attempt to do that for every setting that might get missed bu Ops Management folk... (or perhaps I should say, for as many settings as possible - for some it is clearly not appropriate to have the system make a default decision...)

in general a custom exception is not a lot of effort... here's an example...

[Serializable]
public class MyCustomApplicationException : ApplicationException
{
    #region privates
    #endregion privates

    #region properties
    #endregion properties

    public MyCustomApplicationException (string sMessage,
        Exception innerException)
        : base(sMessage, innerException) { }
    public MyCustomApplicationException (string sMessage)
        : base(sMessage) { }
    public MyCustomApplicationException () { }

    #region Serializeable Code
    public MyCustomApplicationException (
       SerializationInfo info, StreamingContext context)
        : base(info, context) { }
    #endregion Serializeable Code
}