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

.net, asp.net-mvc-3, razor, razor-declarative-helpers

Solution

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

@helper TestHelperMethod(HtmlHelper html) {
    var extra = "class=\"foo\"";
    <div@html.Raw(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()

Problem

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.

Original source

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