Immutable constructor argument order issues result in NullReferenceException

For example:

    class FooOptions
    {
        public FooOptions(string option, Uri foo)
        {
            Foo = foo;
            Option = option;
        }

        [Option("foo")]
        public Uri Foo { get; }    

        [Option("option")]
        public string Option { get;  }
    }

Will result in:

nhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at CommandLine.Infrastructure.ReflectionHelper.CreateDefaultImmutableInstance(Type type, Type[] constructorTypes)
   at CommandLine.Core.ReflectionExtensions.AutoDefault(Type type)
   at CommandLine.Core.InstanceChooser.<>c__DisplayClass1_0.<MatchVerb>b__1()
   at CommandLine.Core.InstanceBuilder.<>c__0`1.<Build>b__0_0(Func`1 f)
   at CSharpx.MaybeExtensions.MapValueOrDefault[T1,T2](Maybe`1 maybe, Func`2 func, T2 noneValue)
   at CommandLine.Core.InstanceBuilder.Build[T](Maybe`1 factory, Func`3 tokenizer, IEnumerable`1 arguments, StringComparer nameComparer, Boolean ignoreValueCase, CultureInfo parsingCulture, IEnumerable`1 nonFatalErrors)
   at CommandLine.Core.InstanceChooser.MatchVerb(Func`3 tokenizer, IEnumerable`1 verbs, IEnumerable`1 arguments, StringComparer nameComparer, CultureInfo parsingCulture, IEnumerable`1 nonFatalErrors)
   at CommandLine.Core.InstanceChooser.<>c__DisplayClass0_0.<Choose>b__0()
   at CommandLine.Core.InstanceChooser.Choose(Func`3 tokenizer, IEnumerable`1 types, IEnumerable`1 arguments, StringComparer nameComparer, CultureInfo parsingCulture, IEnumerable`1 nonFatalErrors)
   at CommandLine.Parser.ParseArguments(IEnumerable`1 args, Type[] types)
   at CommandLine.ParserExtensions.ParseArguments[T1,T2](Parser parser, IEnumerable`1 args)
   at CommandLineParserTest.Program.Main()

The ideal solution IMO would be to find the right ctor by param names (rather than order). But even if that’s not implemented, a better and more meaningful exception would be very helpful in debugging such issues.

[ohadschn]

Agreed. BTW, this can get even more confusing when option class inheritance is in play. Spoiler: the inherited options (from the base options class) are expected at the end of the constructor parameter list.

[ohadschn]

@moh-hassan I would add some text like:

Note that constructor argument order must match property definition order in the options class (inherited options are expected at the end of the constructor parameter list).

Otherwise a client may not understand why no matching constructor was found…