.NET Core 3 Custom Validation Classnames

_ValidationScriptsPartial.cshtml

// This is the default partial generated in the template, if you have another place
// where you include the jQuery unobtrusive validation plugin, put this code there

@using Foo
@inject IOptions<ValidationOptions> validationOptionsHolder
@{ var validationOptions = validationOptionsHolder.Value; }

// Here is your normal environment stuff
  
<script>
    (function () {
        // Set default options for the validation
        const defaultOptions = {
            errorClass: "@validationOptions.InvalidInput", // or add custom, extra ones
            validClass: "@validationOptions.ValidInput"
        };

        $.validator.setDefaults(defaultOptions);
        $.validator.unobtrusive.options = defaultOptions;
    })();
</script>

CustomHtmlGenerator.cs

using System.Collections.Generic;
using System.Text.Encodings.Web;
using Microsoft.AspNetCore.Antiforgery;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.Routing;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.Extensions.Options;

namespace Foo
{
    // Custom HTML Generator to override the default behaviour and replace the CSS classes as they emerge
    public class CustomHtmlGenerator : DefaultHtmlGenerator
    {
        private readonly ValidationOptions _options;

        public CustomHtmlGenerator(IAntiforgery antiforgery, IOptions<MvcViewOptions> optionsAccessor,
            IModelMetadataProvider metadataProvider, IUrlHelperFactory urlHelperFactory, HtmlEncoder htmlEncoder,
            ValidationHtmlAttributeProvider validationAttributeProvider, IOptions<ValidationOptions> options) : 
            base(antiforgery, optionsAccessor, metadataProvider, urlHelperFactory, htmlEncoder, validationAttributeProvider)
        {
            _options = options.Value;
        }

        protected override TagBuilder GenerateInput(ViewContext viewContext, InputType inputType, ModelExplorer modelExplorer, string expression,
            object value, bool useViewData, bool isChecked, bool setId, bool isExplicitValue, string format,
            IDictionary<string, object> htmlAttributes)
        {
            var tagBuilder = base.GenerateInput(viewContext, inputType, modelExplorer, expression, value, useViewData, isChecked, setId, isExplicitValue, format, htmlAttributes);
            FixValidationCssClassNames(tagBuilder);

            return tagBuilder;
        }
        
        public override TagBuilder GenerateTextArea(ViewContext viewContext, ModelExplorer modelExplorer, string expression, int rows,
            int columns, object htmlAttributes)
        {
            var tagBuilder = base.GenerateTextArea(viewContext, modelExplorer, expression, rows, columns, htmlAttributes);
            FixValidationCssClassNames(tagBuilder);

            return tagBuilder;
        }

        public override TagBuilder GenerateValidationMessage(ViewContext viewContext, ModelExplorer modelExplorer, string expression,
            string message, string tag, object htmlAttributes) 
        {
            var tagBuilder = base.GenerateValidationMessage(viewContext, modelExplorer, expression, message, tag, htmlAttributes);
            FixValidationCssClassNames(tagBuilder);

            return tagBuilder;
        }

        public override TagBuilder GenerateValidationSummary(ViewContext viewContext, bool excludePropertyErrors, string message,
            string headerTag, object htmlAttributes)
        {
            var tagBuilder = base.GenerateValidationSummary(viewContext, excludePropertyErrors, message, headerTag, htmlAttributes);
            FixValidationCssClassNames(tagBuilder);

            return tagBuilder;
        }

        private void FixValidationCssClassNames(TagBuilder tagBuilder)
        {
            tagBuilder.ReplaceCssClass(HtmlHelper.ValidationInputCssClassName, _options.InvalidInput);
            tagBuilder.ReplaceCssClass(HtmlHelper.ValidationInputValidCssClassName, _options.ValidInput);
            tagBuilder.ReplaceCssClass(HtmlHelper.ValidationMessageCssClassName, _options.InvalidMessage);
            tagBuilder.ReplaceCssClass(HtmlHelper.ValidationMessageValidCssClassName, _options.ValidMessage);
            tagBuilder.ReplaceCssClass(HtmlHelper.ValidationSummaryCssClassName, _options.InvalidSummary);
            tagBuilder.ReplaceCssClass(HtmlHelper.ValidationSummaryValidCssClassName, _options.ValidSummary);
        }
    }

    public static class TagBuilderHelpers
    {
        public static void ReplaceCssClass(this TagBuilder tagBuilder, string old, string val)
        {
            if (!tagBuilder.Attributes.TryGetValue("class", out string str)) return;
            tagBuilder.Attributes["class"] = str.Replace(old, val);
        }
    }
}

Startup.cs

using System;
using System.DirectoryServices.AccountManagement;
using System.Linq;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Authentication.WsFederation;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc.DataAnnotations;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Options;

namespace Foo
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            // All your usual stuff
            
            // Configure Options
            services.Configure<ValidationOptions>(Configuration.GetSection("Validation")); // or use a custom source

            // Configure custom html generator to override css classnames
            services.AddSingleton<IHtmlGenerator, CustomHtmlGenerator>();
        }
    }
}

ValidationOptions.cs

namespace Foo
{
    // Options class to set the validation classes. Of course, this is not required as you can
    // also harcode the classnames in the CustomHtmlGenerator file.
    public class ValidationOptions
    {
        public string ValidInput { get; set; }
        public string InvalidInput { get; set; }
        public string InvalidMessage { get; set; }
        public string ValidMessage { get; set; }
        public string InvalidSummary { get; set; }
        public string ValidSummary { get; set; }
    }
}

I think you'll have to dig into the inner working of the validator then... Not sure how that could be done.

I ultimately figured it out... the values are hard-coded in jquery.validate.unobtrusive.js so a direct change is required. I added a couple of getter methods to pull the class name from the options from '$jQval.unobtrusive.options', then replaced all the hard coded values with calls to the getter functions. Fingers crossed MS does something better in .NET 8!

Thanks again!

    function getFieldValidationErrorCss() {
        if ($jQval.unobtrusive && $jQval.unobtrusive.options) {
            if ($jQval.unobtrusive.options.fieldInvalidClass)
                return $jQval.unobtrusive.options.fieldInvalidClass;
        }
        
        return "field-validation-error";
    }

    function getFieldValidationValidCss() {
        if ($jQval.unobtrusive && $jQval.unobtrusive.options) {
            if ($jQval.unobtrusive.options.fieldValidClass)
                return $jQval.unobtrusive.options.fieldValidClass;
        }

        return "field-validation-valid";
    }