BigJoe714 BigJoe714 - 1 year ago 113
C# Question

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

I was just writing a console utility and decided to use NDesk.Options for command-line parsing. My question is, How do I enforce required command-line options?

I see in the docs that:

options with a required value (append '=' to the option name) or an optional value (append ':' to the option name).

However, when I put a
at the end of the option name there is no difference in behavior. Ideally the Parse method would throw an exception.

Is there something else I need to do?

Here is my test code:

class Program
static void Main(string[] args)
bool show_help = false;
string someoption = null;

var p = new OptionSet() {
{ "someoption=", "Some String Option", v => someoption = v},
{ "h|help", "show this message and exit", v => show_help = v != null }

List<string> extra;
extra = p.Parse(args);
catch (OptionException e)
System.Console.Write("myconsole: ");
System.Console.WriteLine("Try `myconsole --help' for more information.");

if (show_help)


static void ShowHelp(OptionSet p)
System.Console.WriteLine("Usage: myconsole [OPTIONS]");

Answer Source

The problem is that documentation isn't as clear as it apparently needs to be. :-(

Specifically, as per:

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:


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");
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