Best way to get a property index in ErrorMessage

I’m somewhat struggling with getting the path and in case of collections index of a validated property in the ErrorMessage. I’m using this wonderful library to validate requests coming into Azure functions and I very much do want to return the whole JSON path and index instead of just a property name.

I’ve managed to do so with path using some extension methods to extract the JsonProperty attribute (or fall back to a reflected property name) and passing that to the constructors of my validators and to WithName/WithMessage like this .WithName(x => CreatePath(x.GetPropertyJsonName(nameof(x.Id)))).

I do have a problem with extracting indexes from collections though. Having a parent validator PermissionValidator with a rule:

RuleForEach(x => x.Interests)
               .SetValidator(x => new InterestValidator(CreatePath(x.GetPropertyJsonName(nameof(x.Interests)))));

Where Interests is of type List<Interest> defined like

[Serializable]
[DataContract]
public class Interest
{
    [DataMember, JsonProperty("id")]
    public string Id { get; set; }

    [DataMember, JsonProperty("name")]
    public string Name { get; set; }
}

Is it possible to include the index of the currently validated Interest object in the message coming from InterestValidator? So instead of getting a message saying permission.interest.name cannot be empty it would be permission.interest[0].name cannot be empty.

Sorry if it’s a dumb question, this is my first time using FluentValidation and I can’t really find any examples of this behavior.

[JeremySkinner]

So I was thinking something like this:

public static IRuleBuilderOptions<T,TProperty> CustomPlaceholder<T,TProperty>(this IRuleBuilderOptions<T, TProperty> ruleBuilder, string name, Func<T, object> selector) {
	return ruleBuilder.Configure(cfg => {

		var originalMessageBuilder = cfg.MessageBuilder;
		
		cfg.MessageBuilder = context => {
			context.MessageFormatter.AppendArgument(name, selector((T) context.Instance));
			return originalMessageBuilder != null ? originalMessageBuilder(context) : context.GetDefaultMessage();
		};

	});
}

RuleFor(x => x.Name).NotNull().WithMessage("{SomePlaceholder}").CustomPlaceholder("SomePlaceholder", x => x.SomeProp);

[JeremySkinner]

And just to clarify, the instance being validated is accessible from the MessageBuilder’s context (context.Instance)