TagHelpers add custom class for LabelTagHelper based on validation attribute [Required]

Question

In Core MVC there is anew concept as Tag helpers.

We could previously create custom html helpers to attach some classes based on the validation data annotations such

Answer 1

Yup, you can extend this pretty easily by inheriting from the LabelTagHelper class and adding in your own class to the attribute list first.

[HtmlTargetElement("label", Attributes = "asp-for")]
public class RequiredLabelTagHelper : LabelTagHelper
{
    public RequiredLabelTagHelper(IHtmlGenerator generator) : base(generator)
    {
    }

    public override Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        if (For.Metadata.IsRequired)
        {
            CreateOrMergeAttribute("class", "required", output);
        }

        return base.ProcessAsync(context, output);
    }

    private void CreateOrMergeAttribute(string name, object content, TagHelperOutput output)
    {
        var currentAttribute = output.Attributes.FirstOrDefault(attribute => attribute.Name == name);
        if (currentAttribute == null)
        {
            var attribute = new TagHelperAttribute(name, content);
            output.Attributes.Add(attribute);
        }
        else
        {
            var newAttribute = new TagHelperAttribute(
                name,
                $"{currentAttribute.Value.ToString()} {content.ToString()}",
                currentAttribute.ValueStyle);
            output.Attributes.Remove(currentAttribute);
            output.Attributes.Add(newAttribute);
        }
    }
}