.net - 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

Using Drew Noakes suggestion, I have come to a workaround that does the trick for now and that can be easily removed once the issue is solved in a newer version of MVC (that is if more stuff isn't changed that would break it:))

The goal is to be able to use an HtmlHelper inside a declarative helper method that lives in a file in App_Code without having a NullReferenceException. To solve this I included in all the files in App_Code the following:

@using System.Web.Mvc;

@functions
{
    private static new HtmlHelper<object> Html
    {
        get { return ((WebViewPage)CurrentPage).Html; }
    }

    private static UrlHelper Url
    {
        get { return ((WebViewPage)CurrentPage).Url; }
    }
}

This does seem to do the trick as I can now write the following helper (in the same file):

@helper PrintAsRaw(string htmlString)
{
     @Html.Raw(htmlString)
}

Obviously the helper method is just an example. This solution has the downside that the @functions declarations has to be introduced in all helper declaration files in App_Code, but does avoid complicating the call to the helper, as you can simply write in a view:

@MyAppCodeFile.PrintAsRaw("<p>My paragraph</p>")

Hope this helps...