How to write HtmlHelper in Fluent syntax - asp.net-mvc

Question

I have a simple tag builder that looks like this:
public static MvcHtmlString Tag(this HtmlHelper helper, string tag, string content)
{
var tagBuilder = new TagBuilder(tag){InnerHtml = content};
return MvcHtmlString.Create(tagBuilder.ToString(TagRenderMode.NormalTag));
}
And, I can use it like this:
#Html.Tag("em", Model.Title)
which produces:
<em>The Title</em>
How can this be written to use a Fluent Syntax so it's use would look like this:
#Html.Tag("em").Content(Model.Title)

Answer 1

You have to define a builder interface and implementation. I hope my example can provide some guidance:
public static class MyHtmlExtensions
{
public static IMyTagBuilder Tag(this HtmlHelper helper, string tag)
{
return new MyTagBuilder(tag);
}
}
Then you define your builder interface and implementation:
public interface IMyTagBuilder : IHtmlString
{
IHtmlString Content(string content);
}
public class MyTagBuilder : IMyTagBuilder
{
private readonly TagBuilder _tagBuilder;
public MyTagBuilder(string tag)
{
_tagBuilder = new TagBuilder(tag);
}
public IHtmlString Content(string content)
{
_tagBuilder.InnerHtml = content;
return this;
}
public string ToHtmlString()
{
return _tagBuilder.ToString(TagRenderMode.NormalTag);
}
}
Since IMyTagBuilder implements IHtmlString, it can be used either with or without calling .Content() afterwards.
A great trick to use when implementing fluent interfaces it to use a IFluentInterface to hide object members (ToString, Equals, GetHashCode and GetType) from IntelliSense, it removes some noise.
EDIT: A great resource for building fluent APIs is Daniel Cazzulino's screencast from building Funq here