ASP.NET Core TagHelper to load element attributes from ViewData.

HtmlElementAttributeTagHelper.cs

using System;
using System.Collections.Generic;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Razor.TagHelpers;

namespace AspNetCore.Helpers
{
    [HtmlTargetElement(Attributes = ViewBagKeyAttributeName)]
    public class HtmlElementAttributeTagHelper : TagHelper
    {
        private const string ViewBagKeyAttributeName = "asp-attributes-key";

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

        [HtmlAttributeName(ViewBagKeyAttributeName)]
        public string AttributesKey { get; set; }

        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            if (!string.IsNullOrWhiteSpace(AttributesKey))
            {
                var attributes = ViewContext.ViewData[AttributesKey];
                switch (attributes)
                {
                    case IDictionary<string, object> dict:
                    {
                        foreach (var attr in dict)
                        {
                            output.Attributes.Add(attr.Key, attr.Value);
                        }
                        break;
                    }
                    case object obj:
                    {
                        foreach (var attr in HtmlHelper.AnonymousObjectToHtmlAttributes(obj))
                        {
                            output.Attributes.Add(attr.Key, attr.Value);
                        }
                        break;
                    }
                }
            }
        }
    }
}