How to add attribute without value

Question

I am generating HTML textbox through the html helper and TagBuilder.

we have the method TagBuilder.Attributes.Add("key","value")

but for HTML5 required attribute does not need value to be passed, so if i pass empty string then the output with value of required = ""

So how do i add required attribute without passing the value?

 public static IHtmlString AppTextBox(this HtmlHelper helper, string model)
    {
        var input = new TagBuilder("input");
        input.Attributes.Add("class", "form-control");
        input.Attributes.Add("ng-model", model);

        input.Attributes.Add("required","");

        return new MvcHtmlString(input.ToString(TagRenderMode.Normal));
    }

Answer 1

It's also valid to pass the name of the attribute as the value:

input.Attributes.Add("required", "required");

Answer 2

I've tested on MVC 5, not sure about older versions but the following does what you want.

tagBuilder.MergeAttribute("required", string.Empty);