Preprocessing Markdown Documents for Keywords

I recently added a feature to my application to serve arbitrary markdown documents from a directory. This is designed to allow authors to populate a folder full of help documents, and be able to view them without any code changes. However, because this is an MVC application, I needed a few extra features. Straight to it, here's my controller:

public class DocumentationController : Controller
{
    private const string DefaultDocPage = "Overview";
    public ActionResult Index(string id = null)
    {
        ViewBag.HomeLinkPage = string.IsNullOrEmpty(id) || id == DefaultDocPage ? string.Empty : DefaultDocPage;
        if (string.IsNullOrEmpty(ViewBag.HomeLinkPage))
        {
            id = DefaultDocPage;
        }
        var filePath = Server.MapPath(Url.Content("~/Content/Documentation/" + id.Trim("/".ToCharArray()) + ".md"));
        if(!System.IO.File.Exists(filePath))
        {
            Response.StatusCode = (int)HttpStatusCode.NotFound;
            return null;
        }
        var contents = new StringBuilder(System.IO.File.ReadAllText(filePath));

        contents.Replace("{{AppRoot}}/", Url.Content("~"));
        contents.Replace("{{Documentation}}", Url.Content("~/Documentation"));
        contents.Replace("{{DocumentationImages}}", Url.Content("~/Content/Documentation/images"));
        contents.Replace("{{SupportLink}}", ConfigurationManager.AppSettings["SupportLink"]);
        return View("Index", (object)contents.ToString());
    }
}

In short, it allows passing a file name (without ".md") called "id" (poor name is my fault for lazy routing), assigns a default document if a file name was not passed, sets a ViewBag link to the homepage if we're not on the default document, maps the file name to a markdown document in the directory, replaces some preprocessed keywords, and returns the view with the contents of the document.

My view renders the homepage link, if applicable, then the document using MarkdownSharp. I haven't seen my idea of

A few minor nit-picks:

-
I'd change the id checking code around a bit to avoid some redundant checks by first sanitizing the input:

id = string.IsNullOrEmpty(id) ? DefaultDocPage : id;
ViewBag.HomeLinkPage = id == DefaultDocPage ? string.Empty : DefaultDocPage;

-
Consider making DefaultDocPage a configurable property so you don't have to change the code if when someone asks to have a different name

-
Trim takes a params array so there is no need to do the ToCharArray, you can just pass in all the trim characters directly like this:

id.Trim('/')

-
Replacing

var filePath = Server.MapPath(Url.Content("~/Content/Documentation/" + id.Trim("/".ToCharArray()) + ".md"));

with string.format makes it a bit easier to read:

var filePath = Server.MapPath(Url.Content(string.format("~/Content/Documentation/{0}.md", id.Trim('/'))));

1. There is no need to cast to object here:

return View("Index", (object)contents.ToString());

StackExchange Code Review Q#45877, answer score: 2