Why is the HtmlHelper instance null in a Razor declarative @helper method?

Question

Using MVC 3 RTM I'm getting a strange NullReferenceException:

@helper TestHelperMethod() {
    var extra = "class=\"foo\"";
    <div @Html.Raw(extra)></div>
}

It turns out that Html (of type HtmlHelper) is null.

I've never seen this before in a regular view. I'm starting to experiment with declarative helper methods in Razor (so far they seem a little limited) and I'm quite stumped by what I'm seeing here.

Answer 1

That's a known limitation of those helpers. One possibility is to pass it as parameter:

@helper TestHelperMethod(HtmlHelper html) {
    var extra = "class=\"foo\"";
    <[email protected](extra)></div>
}

Another possibility is to write the helper as an extension method:

public static class HtmlExtensions
{
    public static MvcHtmlString TestHelperMethod(this HtmlHelper)
    {
        var div = new TagBuilder("div");
        div.AddCssClass("foo");
        return MvcHtmlString.Create(div.ToString());
    }
}

and then:

@Html.TestHelperMethod()