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;
try
{
extra = p.Parse(args);
}
catch (OptionException e)
{
System.Console.Write("myconsole: ");
System.Console.WriteLine(e.Message);
System.Console.WriteLine("Try `myconsole --help' for more information.");
return;
}
if (show_help)
{
ShowHelp(p);
return;
}
System.Console.WriteLine("==================");
System.Console.WriteLine(someoption);
}
static void ShowHelp(OptionSet p)
{
System.Console.WriteLine("Usage: myconsole [OPTIONS]");
System.Console.WriteLine();
System.Console.WriteLine("Options:");
p.WriteOptionDescriptions(System.Console.Out);
}
}
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:
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");