namespace ZKEACMS.Message.TagHelps
{
 [HtmlTargetElement("markdown", TagStructure = TagStructure.NormalOrSelfClosing)]
 [HtmlTargetElement(Attributes = "markdown")]
 public class MarkdownTagHelper : TagHelper
 {
  public ModelExpression Content { get; set; }
  public async override Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
  {
   if (output.TagName == "markdown")
   {
    output.TagName = null;
   }
   output.Attributes.RemoveAll("markdown");
   var content = await GetContent(output);
   var markdown = WebUtility.HtmlEncode(WebUtility.HtmlDecode(content));
   var html = CommonMarkConverter.Convert(markdown);
   output.Content.SetHtmlContent(html ?? "");
  }
  private async Task GetContent(TagHelperOutput output)
  {
   if (Content == null)
    return (await output.GetChildContentAsync()).GetContent();
   return Content.Model?.ToString();
  }
 }
}

@addTagHelper *, ZKEACMS.Message

<markdown>@item.CommentContent</markdown>