What is the most appropriate .NET exception to throw upon failing to load an expected registry setting?

Question

I have an application which tries to load some expected registry settings within its constructor.

What is the most appropriate .NET Exception from the BCL to throw if these (essential, non-defaultable) registry settings cannot be loaded?

For example:

 RegistryKey registryKey = Registry.LocalMachine.OpenSubkey("HKLM\Foo\Bar\Baz");

 // registryKey might be null!
 if (registryKey == null)
 {
     // What exception to throw?
     throw new ???Exception("Could not load settings from HKLM\foo\bar\baz.");
 }

Answer 1

Why not create your custom exception?

public class KeyNotFoundException : RegistryException
{
    public KeyNotFoundException(string message)
        : base(message) { }
}
public class RegistryException : Exception
{
    public RegistryException(string message)
        : base(message) { }
}

....

if (registryKey == null)
{
    throw new KeyNotFoundException("Could not load settings from HKLM\foo\bar\baz.");
}

Also, instead of inheriting from Exception you could inherit from ApplicationException. This depends on the kind of failure you want your application to have in this situation.

Answer 2

actually, I wouldn't throw an exception here. I would have a default value, and then create the key using that default value.

If you MUST have a user-defined value, I'd use the ArgumentException (as that's fundamentally what you're missing, an argument for your constructor--where you store it is irrelevant to the type of exception you're trying to generate).

Answer 3

I'd go with ArgumentException or ArgumentOutOfRangeException..

throw new ArgumentException("Could not find registry key: " + theKey);

Quoting MSDN:

The exception that is thrown when one of the arguments provided to a method is not valid.

...

IMO writing a proper exception message is more important.

Answer 4

It depends on why it failed. If it's a permissions issue, the I'd go with System.UnauthorizedAccess exception:

The exception that is thrown when the operating system denies access because of an I/O error or a specific type of security error.

I don't know if it matches the "specific type", but it is a security error, and access wasn't authorized.

On the other hand, if the item just doesn't exist then I'd thrown a FileNotFound exception. Of course, a registry key isn't a file, but FileNotFound is pretty well understood.