Validate that two or more options are compatible with each other

[bjorg]

I know how to validate individual options, but I have a use case where I have 2 options that need to be validated together.

Take the following example where the command takes 2 dates. Each date option has a default value when omitted. The command needs to make sure that the dates are valid with each other.

How do I do that?

var startDateOption = new Option<DateOnly>("--start-date") {
    Description = "Start date for evaluation",
    DefaultValueFactory = _ => DateOnly.Parse("2018-01-01")
};
var endDateOption = new Option<DateOnly>("--end-date") {
    Description = "End date for evaluation",
    DefaultValueFactory = _ => DateOnly.Parse("2022-12-31")
};
var rootCommand = new RootCommand("root") {
    startDateOption, 
    endDateOption 
};
rootCommand.SetAction(parseResult => {
    var startDate = parseResult.GetValue(startDateOption);
    var endDate = parseResult.GetValue(endDateOption);
    if(startDate > endDate) {
        throw new Exception($"{startDateOption.Name} must be before {endDateOption.Name}");
    }
    // do work
    return 0;
}

I have tried using a validator on the command, but it appears to be executed before the options are parsed, so I don't have access to the value created by DefaultValueFactory.

The above code is the best approximation I have, but I don't see a way to generate a validation error that late in the execution flow when inside the action handler.

[jonsequitur]

Give Option<T>.CustomParser a look. It lets you do validation and default value generation all at once.

[bjorg]

Thank for the suggestion. I've used Option<T>.CustomParser for another option in my app, but I don't see how I could reference the parsed result of startDateOption from endDateOption. As far as I can tell, having a reference to the option is not good enough. That's why validators don't work either in this scenario.

Frankly, I would be fine with my solution doing it in the command action, if there was a way to report an error like validators or custom parsers, but I don't think that's possible.