A first chance exception message gets written to the Visual Studio Output window when the debugger is given the first chance to see/handle the exception (hence the name). If the debugger is not configured to stop on such an exception then execution will
continue. In this case the exception is being caught in code and so execution will continue. The source of your Output window message comes from this block of code in the ConfigurationManager:
Enum.Parse(typeof(Lexer), _language, true);
// If we made it here, the language matches one of
// the lexer names, just use that.
s = _language;
// No match, oh well. Don't set the lexer.
As you can see there is an attempt to match the string passed to ConfigurationManager.Lanaguage property to the Lexer enumeration. Trying to parse an empty string is what causes the exception. You can also see that it's currently being swallowed -- obviously
not a good practice and a piece of code that should be rewritten to look for empty/null strings before attempting to parse. I'll put it on my to-do list, but CodePlex also makes it very easy to submit a patch yourself. :)
As a side note, it's easy to determine the source of first chance exceptions in Visual Studio by setting a breakpoint on an exception. You can do that by opening the Exception dialog from the Debug -> Exceptions menu item. Place a check in the "Thrown"
column of the System.ArgumentException exception and your debugger will stop anytime one of these types of exceptions occurs again.