HttpAbstractValidator.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

using FluentValidation;
using FluentValidation.AspNetCore;
using FluentValidation.Results;
using Microsoft.AspNetCore.Mvc;

namespace WebApiTest
{
    /// <summary>
    /// This class is intended to be used in WebApi projects, making it possible to
    /// have different validations on POST (where you don't want to send an Id) and other calls.
    /// It can only be used where we have an http context, and the NotEmptyOnUpdate test can only be used
    /// in an HttpAbstractValidator.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public abstract class HttpAbstractValidator<T> : AbstractValidator<T>, IValidatorInterceptor
    {
        public ValidationResult AfterMvcValidation(ControllerContext controllerContext, IValidationContext commonContext, ValidationResult result)
        {
            return result;
        }

        public IValidationContext BeforeMvcValidation(ControllerContext controllerContext, IValidationContext commonContext)
        {
            var a = controllerContext?.HttpContext?.Request?.Method;
            commonContext.RootContextData["HttpMethod"] = a;
            return commonContext;
        }
    }

    public static class ExtensionMethods
    {
        public static IRuleBuilderOptions<T, TElement> NotEmptyOnUpdate<T, TElement>(this IRuleBuilder<T, TElement> ruleBuilder)
        {
            return ruleBuilder.Must((rootObject, field, context) => {
                if (context.ParentContext.RootContextData.ContainsKey("HttpMethod") == false) 
                    throw new ValidationException("NotEmptyOnUpdate validation can only be used by a HttpAbstractValdator");

                string method = context.ParentContext.RootContextData["HttpMethod"] as string;                
                context.MessageFormatter.AppendArgument("HttpMethod", method);
                if (method == "POST")
                    return true; // Bypass check
                else
                    return !string.IsNullOrEmpty(field as string);
            })
            .WithMessage("{PropertyName} must not be empty in a {HttpMethod} request.");
        }
    }
}