Why Option exposes IsRequired while it also exposes Arity?

[adamsitnik]

Currently there are two ways of saying that given Option is required:

• by setting option.IsRequired to true
• by setting option.Arity to an arity with MinimumNumberOfValues >= 1.

We also have two different errors for both cases: OptionRequiredArgumentMissing and RequiredOptionWasNotProvided.

I need to understand it better to provide good implementation for #2052

[jonsequitur]

Option.Arity describes how many argument tokens the option requires if the option is provided. It only has significance when the option is present on the command line being parsed.

Option.IsRequired indicates whether an option must be provided. Only when it's provided are the arity rules meaningful.

[adamsitnik]

@jonsequitur how does it work when it comes to Argument, which exposes Arity but no IsRequired?

Currently the RequiredArgumentMissing error is reported for the CommandResult which owns Argument, rather than for the ArgumentResult itself (it's not created at all).
Because of that, ParseResult.FindResultFor(argument) returns null and ParseResult.GetValue(argument) returns default.

Argument<int> argument = new ("arg");
Command command = new("name") { argument };

ParseResult parseResult = command.Parse("");
foreach (var error in parseResult.Errors)
    Console.WriteLine($"{error.Message}, SymbolResult: {error.SymbolResult}");

Console.WriteLine($"Value: {parseResult.GetValue(argument)}");

Required argument missing for command: 'name'., SymbolResult: CommandResult: name
Value: 0

This seems to be by design:

command-line-api/src/System.CommandLine.Tests/Binding/TypeConversionTests.cs

Lines 947 to 960 in c63e231

	[Fact]
	public void String_defaults_to_null_when_not_specified()
	{
	var argument = new Argument<string>("arg");
	var command = new Command("mycommand")
	{
	argument
	};
	var result = command.Parse("mycommand");
	result.GetValue(argument)
	.Should()
	.BeNull();
	}

but it's not intuitive for me (we have a parse error related to given argument, but it's not thrown when value is being requested).

[jonsequitur]

@jonsequitur how does it work when it comes to Argument, which exposes Arity but no IsRequired?

Options are optional by default, whereas arguments are required by default.

The test does look wrong. The default arity for that argument is {1,1} which means there should be an error there.

[adamsitnik]

@jonsequitur big thanks for all the answers, they were very helpful!