Feature request: provide default value for option (and show it in the help)

Apologies if I've missed this one. I see in the examples you're using conditionals to set default values, but what about providing something like

[Option(Description = "Verbosity",DefaultValue = 5)] so when an option is not required a default value is supplied - this could also be reflected in the help text so we're following the Principle of Least Astonishment.

I found https://github.com/natemcmaster/CommandLineUtils/blob/master/samples/Attributes/Program.cs#L19 which is pretty much what I want. I guess the help text can't be easily generated from there.

[liamdawson]

That being said, I would like it if it could infer the default value from the target property, as sometimes I want default values that can be statically set, but can't be set as a constant (e.g. a path that is in the user's home directory).

For example:

[Option("--data-directory")]
public string DataDirectory { get; set; } = Path.Combine(Environment.GetSpecialDirectoryPath(Environment.SpecialDirectories.UserProfile), ".cache/app");

(typed from memory, may not compile)

My hope is that I could add a ShowDefault = true to the OptionAttribute and have it append [default: /home/liamdawson/.cache/app] to the help text.

[dbogatov]

Could we reopen this?
I'm glad the original author has found his workaround, but I would indeed ask that the default value is printed.

[natemcmaster]

Marking this as up-for grabs. I would be okay adding logic to print the default CLR value in the help. I'm not sure yet about adding new properties to OptionAttribute -- I think default property values are a more natural way to express this in C# -- but I would be okay if we added string DefaultValue { get; set; } to CommandOption for the builder API users.

[SeriousM]

My current workaround is to set the default value on the property, don't mark it with Required (as this would stop the execution because the option was not set in the command line) and call Validator.Validate(this, new ValidationContext(this)) at the beginning of OnExecuteAsync to ensure that all the other attrubutes are checked, like FileExists or Url.

[EajksEajks]

And how would you set the default value using the builder API?

[scott-xu]

I made a PR #369 about showing allowed values in help text which is somewhat similar with this issue.

[scott-xu]

Here's my proposal
API:

public class CommandOption
{
   // other members

   public Func<object> DefaultValue { get; set; }
}

Usage
builder:

var app = new CommandLineApplication();
app.Option("-o|--opt", "Some description.", CommandOptionType.SingleValue, o => o.DefaultValue = () => "foo");

attribute:

class Program
{
    [Option("-o|--opt", "Some description.", CommandOptionType.SingleValue)]
    public string? Option { get; } = "foo";
}

help text

Options:
  -o|--opt        Some description.
                  Default value is: foo.

[scott-xu]

We may need re-consider about the below implementation, or leave it as is:

public class CommandOption
{
   // other members

   public bool HasValue()
   {
       return Values.Any();
   }

   public string? Value()
   {
       return HasValue() ? Values[0] : null;
   }
}