title: TagHelper PreContent and PostContent not showing up in HTML when overriding InputTagHelper
body: <p>I want to create a tag helper to put some HTML before and after <code>&lt;input&gt;</code> tags, but I want to keep the default <code>asp-for</code> behavior and access the <code>ModelExpression</code> data.</p>
  
  <p>To this end, I tried to override the <code>Microsoft.AspNetCore.Mvc.TagHelpers.InputTagHelper</code> type, as described <a href="http://rion.io/2017/04/27/extending-tag-helpers-in-asp-net-core-applications/" rel="nofollow noreferrer">in this post</a>. However, even though I can verify in the debugger that the <code>Process</code> method is being called <code>PreContent</code> and <code>PostContent</code> are both being set, nothing shows up in the HTML other than the standard <code>&lt;input&gt;</code> tag. It works fine when creating a tag helper from scratch for another tag though.</p>
  
  <p>I created a small project to demonstrate this issue. I put <a href="https://github.com/MiffOttah/TagHelperIssue" rel="nofollow noreferrer">the entire project on GitHub</a>, and I'm copying the specific tag helper I'm trying to create below.</p>
  
  <pre class="lang-cs prettyprint-override"><code>    [HtmlTargetElement("input", Attributes = "asp-for,test-label")]
      public class TestTagHelper : InputTagHelper
      {
          public TestTagHelper(IHtmlGenerator generator) : base(generator)
          {
          }
  
          [HtmlAttributeName("test-label")]
          public string Label { get; set; }
  
          public override void Process(TagHelperContext context, TagHelperOutput output)
          {
              output.PreContent.SetHtmlContent($"&lt;b&gt;{WebUtility.HtmlEncode(Label)}&lt;/b&gt; ");
              output.PostContent.SetHtmlContent($" &lt;i&gt;({WebUtility.HtmlEncode(For.Name)})&lt;/i&gt;"); // access information from the input tag
              base.Process(context, output);
          }
      }
  </code></pre>
  
  <p>Am I missing something obvious? Is there a caveat to doing this? Or is this something that just can't be done?</p>