Intercepting Fluent Validation - servicestack

Question

We are using fluentvalidation (with service stack) to validate our request DTO's. We have recently extended our framework to accept "PATCH" requests, which means we now have a requirement to apply validation ONLY when the patch contained the field being validated.
We have done this using an extension method such as this:
RuleFor(dto => dto.FirstName).Length(1,30)).WhenFieldInPatch((MyRequest dto)=>dto.FirstName);
RuleFor(dto => dto.MiddleName).Length(1,30)).WhenFieldInPatch((MyRequest dto)=>dto.MiddleName);
RuleFor(dto => dto.LastName).Length(1,30)).WhenFieldInPatch((MyRequest dto)=>dto.LastName);
This means we can run the same validation for a POST/PUT or a PATCH.
I have been looking for a way of hooking in to the fluent validation framework in such as way that we do not need to duplicate the .WhenFieldInPatch() rule on EVERY line in our validations, but have not yet found a nice way to do this.
I have tried the following:
Creating a helper method (in a in a base class) to intercept the initial "RuleFor" which adds the .When() clause up front, but the this does not work as fluent validation requires the .When() to be last
Intercepting the calls in PreValidation, but I can only intercept based on the whole class, and not on a rule by rule basis
Adding an extension method to apply to the end of every rule (as per example), but I cannot access the initial expression in order to check whether the field should be mapped - so I need to pass it in again.
Am I missing something, or am I attempting the impossible?
Thanks

Answer 1

When I need to share Fluent Validation Logic I'd use extension methods, here's an example of shared Extension methods for TechStacks, e.g:
public static class ValidatorUtils
{
public static bool IsValidUrl(string arg) => Uri.TryCreate(arg, UriKind.Absolute, out _);
public static string InvalidUrlMessage = "Invalid URL";
public static IRuleBuilderOptions<T, string> OptionalUrl<T>(
this IRuleBuilderInitial<T, string> propertyRule)
{
return propertyRule
.Length(0, UrlMaxLength)
.Must(IsValidUrl)
.When(x => !string.IsNullOrEmpty(x as string))
.WithMessage(InvalidUrlMessage);
}
}
And some examples where they're shared:
public class CreatePostValidator : AbstractValidator<CreatePost>
{
public CreatePostValidator()
{
RuleSet(ApplyTo.Post, () =>
{
RuleFor(x => x.Url).OptionalUrl();
});
}
}
public class UpdatePostValidator : AbstractValidator<UpdatePost>
{
public UpdatePostValidator()
{
RuleSet(ApplyTo.Put, () =>
{
RuleFor(x => x.Url).OptionalUrl();
});
}
}