asp.net core tag helper for conditionally add class to an element

Question

In Asp.Net MVC we can add class conditionally as following code:

Answer 1

Ability to add a conditional css class by following tagHelper provides. this code like AnchorTagHelper asp-route-* for add route values acts.

[HtmlTargetElement("div", Attributes = ClassPrefix + "*")]
public class ConditionClassTagHelper : TagHelper
{
    private const string ClassPrefix = "condition-class-";

    [HtmlAttributeName("class")]
    public string CssClass { get; set; }

    private IDictionary _classValues;

    [HtmlAttributeName("", DictionaryAttributePrefix = ClassPrefix)]
    public IDictionary ClassValues
    {
        get {
            return _classValues ?? (_classValues = 
                new Dictionary(StringComparer.OrdinalIgnoreCase));
        }
        set{ _classValues = value; }
    }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        var items = _classValues.Where(e => e.Value).Select(e=>e.Key).ToList();

        if (!string.IsNullOrEmpty(CssClass))
        {
            items.Insert(0, CssClass);
        }

        if (items.Any())
        {
            var classes = string.Join(" ", items.ToArray());
            output.Attributes.Add("class", classes);
        }
    }
}

in _ViewImports.cshtml add reference to taghelper as following

@addTagHelper "*, WebApplication3"

Use tagHelper in View:

result for Active = true and Display = true is: