title: MVC 5 Custom Html Helper extension issue
body: <p>I'm trying to create a custom html helper class.
  I have the below as a very simple start, complies fine:</p>
  
  
  
  <pre class="lang-cs prettyprint-override"><code>using Microsoft.AspNetCore.Mvc.Rendering;
  using Microsoft.AspNetCore.Mvc.ViewFeatures;
  
  namespace Mobile.HtmlHelpers
  {
      public static class RequestBox
      {
          public static HtmlString CascadeBoxFor&lt;TModel, TValue&gt;(this HtmlHelper&lt;TModel&gt; helper, Expression&lt;Func&lt;TModel, TValue&gt;&gt; expression, object htmlAttributes = null)
          {
              HtmlString html = (HtmlString)HtmlHelperInputExtensions.TextBoxFor(helper, expression);
              return html;
          }
          // IHtmlContent html = HtmlHelperInputExtensions.TextBoxFor(htmlHelper, expression);
      }
  }
  </code></pre>
  
  <p>I am trying to call it and the system doesn't like it..
  I can see it if I use:</p>
  
  <pre class="lang-cs prettyprint-override"><code>@foreach (var item in Model.RequestModel.Requests)
  {     
      @RequestBox.CascadeBoxFor(x=&gt;item.EmployeeDescription)
  }
  </code></pre>
  
  <p>But I get:</p>
  
  <blockquote>
    <p>Severity Code Description Project File Line Suppression State
    Error CS7036 There is no argument given that corresponds to the required formal parameter 'expression' of 'RequestBox.CascadeBoxFor&lt;TModel, TValue>(HtmlHelper, Expression&lt;Func&lt;TModel, TValue>>, object)' Mobile..NET Framework 4.6.1 </p>
  </blockquote>
  
  <p>If I try suggestions of it being an extension and I should be able to use <code>@Html.CascadeBoxFor(x =&gt; item.EmployeeDescription)</code>, I get:</p>
  
  <blockquote>
    <p>Severity Code Description Project File Line Suppression State
    Error CS1061 'IHtmlHelper&lt;RequestPageModel>' does not contain a definition for 'CascadeBoxFor' and no extension method 'CascadeBoxFor' accepting a first argument of type 'IHtmlHelper&lt;RequestPageModel>' could be found (are you missing a using directive or an assembly reference?)    Mobile..NET Framework 4.6.1 </p>
  </blockquote>
  
  <p>Can anyone tell me what is missing here?</p>