Question

In the following example we have two different exceptions we want to communicate.

//constructor
public Main(string arg){
   if(arg==null)
      throw new ArgumentNullException("arg");

  Thing foo=GetFoo(arg);

  if(foo==null)
     throw new NullReferenceException("foo is null");    
}

Is this the proper approach for both exception types?

Answer 1

The first exception is definitely correct. It’s the second one which is tricky.

There are two possibilities here:

• GetFoo() isn’t meant to return null, ever. In that case we’ve basically proven a bug in GetFoo(). I’m not sure of the best exception here, leaving ContractException (from Code Contracts) aside. Basically you want something like ContractException – an exception which means “The world has gone crazy: this isn’t just an externally unexpected result, there’s a bug here.”
• GetFoo() can legitimately return null, due to arg‘s value. In this case I would suggest that ArgumentException (but not ArgumentNullException) may be appropriate. On the other hand, it’s odd to throw ArgumentException after using the argument.

InvalidOperationException isn’t quite appropriate here, but I might be tempted to use it as the closest thing to a contract failure…

EDIT: You should also consider creating your own exception, as per Aaronaught’s answer.