How to create optional arguments? about docopt.net HOT 10 CLOSED

Hi currently on holidays with no access to my computer. I will have a look
next week when I am back.

I seem unable to create optional arguments. Many examples on the net show
things such as:

my_app [OPTIONAL]

But when I do this I get a this code generated:

public string ArgOptional { get { return _args["OPTIONAL"].ToString(); } }

So if I don't pass the OPTIONAL argument as soon as I try to access it I
get an exception due to ToString being called on null. As I want this
argument to be optional I expect it to return null or String.Empty not to
throw!?

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#14.

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This is a valid point @jtu100. The default for a string value should be an empty string rather than null.

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Just to clarify the override of ToString() in ValueObject should handle this case so I need to check why it does not work.

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Problem I found is that the reference to ValueObject is itself null so modifying ToString() will not help.
I resolved the problem in Options.GenerateCode like this:
return string.Format("public string {0} {{ get {{ return _args[\"{1}\"] == null ? string.Empty : _args[\"{1}\"].ToString(); }} }}", s, Name);

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My preference would be that the value of an argument never be null, instead have a ValueObject that represents that case. Since that is a breaking change, a reasonable solution might be to add some sort of helper method to check if the value of an arg is null and return a ValueObject with no value if so. Perhaps an extension method for ValueObject?

public static string OptionalValue(this ValueObject self)
{
    return self ?? new ValueObject();
}

Then I can easily write:

string optionalValue = arguments[param].OptionalValue();

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@jcludwig An extension method makes sense. @geoff5777 a PR is welcome if you have one.

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The extension method solution is a good solution but I can't see how it helps for those that are using the T4 generated class. At the very least one would still have to modify Options.GenerateCode to use the extension method.

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I'm also getting stuck on this issue, maybe it would be better to generate helper methods so we can check for optional options without it crashing, and without having to duplicate the flags.

eg. I'm currently using an added method to my T4 file (which is not ideal)

public bool IsProvided(string parameter)
{
    return _args[parameter] != null;
}

but my code ends up with:

var outputFile = args.IsProvided("--output") ? args.OptOutput : string.Empty;

it would be nicer to be able to say:

var outputFile = args.HasOptOutput ? args.OptOutput : string.Empty;

(still would be much nice to just say var outputFile = args.OptOutput ?? string.Empty; though)

Does the IsTrue and AsList generated versions have the same NullReferenceException issue?

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Should this issue be closed?

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yep

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