Tag helper not being processed in ASP.NET Core 2

Question

I've added the following tag helper:

using System;
using System.Linq;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.TagHelpers;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Razor.TagHelpers;

namespace X.TagHelpers
{
    [HtmlTargetElement(Attributes = ValidationForAttributeName + "," + ValidationErrorClassName)]
    public class ValidationClassTagHelper : TagHelper
    {
        private const string ValidationForAttributeName = "k-validation-for";
        private const string ValidationErrorClassName = "k-error-class";

        [HtmlAttributeName(ValidationForAttributeName)]
        public ModelExpression For { get; set; }

        [HtmlAttributeName(ValidationErrorClassName)]
        public string ValidationErrorClass { get; set; }

        [HtmlAttributeNotBound]
        [ViewContext]
        public ViewContext ViewContext { get; set; }

        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            Console.WriteLine("\n\n------------!!!!!!---------\n\n");
            ModelStateEntry entry;
            ViewContext.ViewData.ModelState.TryGetValue(For.Name, out entry);
            if (entry == null || !entry.Errors.Any()) return;
            var tagBuilder = new TagBuilder(context.TagName);
            tagBuilder.AddCssClass(ValidationErrorClass);
            output.MergeAttributes(tagBuilder);
        }
    }
}

and then in _ViewImports.cshtml I've added the line:

@addTagHelper *, X.TagHelpers

The file is compiled correctly and if I introduce a syntax error dotnet build warns me about it.

Then in one of my pages I add:

<div k-validation-for="OldPassword" k-error-class="has-danger"></div>

If I load the page I see no console output on the server side and the k-validation-for and k-error-class are forwarded to the generated page as is (as opposed to adding the has-danger class to the class attribute).

What am I doing wrong?

Answer 1

When registering Tag Helpers, it’s the assembly that is required, not the namespace - explained in the docs.

...the second parameter "Microsoft.AspNetCore.Mvc.TagHelpers" specifies the assembly containing the Tag Helpers. Microsoft.AspNetCore.Mvc.TagHelpers is the assembly for the built-in ASP.NET Core Tag Helpers.

So in your case, you can just change this:

@addTagHelper *, X.TagHelpers

To this:

@addTagHelper *, X