How to enforce required command-line options with NDesk.Options?

The problem is that documentation isn’t as clear as it apparently needs to be. 🙁

Specifically, as per:

http://www.ndesk.org/doc/ndesk-options/NDesk.Options/OptionValueType.html#F:NDesk.Options.OptionValueType.Required

The = within an option specification doesn’t apply to the OptionSet as a whole, but just to the value for that specific option.

The importance of this is really only relevant in two scenarios, so first let’s consider the OptionSet parser:

string a = null;
string b = null;
var options = new OptionSet {
    { "a=", v => a = v },
    { "b=", v => b = v },
};

Scenario 1 where it’s important is that OptionSet.Parse() works in a single-pass, forward-only manner, and does not look at option values to determine if they “should be” values. Thus, consider:

options.Parse(new[]{"-a", "-b"});

The result of this will be that a has the value "-b", and b is null. Since the handler for -a requires a value, it always gets the following value (unless the value is “encoded” into the original option, e.g. -a=value).

The second place where this is important is when a value-requiring option is the last option, and there isn’t a value present for it:

options.Parse(new[]{"-a"});

This will throw an OptionException, as the handler for -a requires a value, and no value is present.

Consequently, if you have an option that itself is required (as opposed to an option that requires a value), you need to manually check for this:

string dir = null;
new OptionSet {
    { "o=", v => dir = v },
}.Parse (args);

if (dir == null)
    throw new InvalidOperationException ("Missing required option -o=DIR");