Plus (+) in MVC Argument causes 404 on IIS 7.0

System.Web.HttpUtility.UrlPathEncode(string str) encodes a + to a %20

+ only has the special meaning of being a space in application/x-www-form-urlencoded data such as the query string part of a URL.

In other parts of the URL like path components, + literally means a plus sign. So resolving Full+Size to the unencoded name Full Size should not work anywhere.

The only correct form of a space in a path component is %20. (It still works when you type an actual space because the browser spots the error and corrects it for you.) %20 also works in form-URL-encoded data as well, so it's generally safest to always use that.

Sadly HttpUtility.UrlEncode is misleadingly-named. It produces + in its output instead of %20, so it's really a form-URL-encoder and not a standard URL-encoder. Unfortunately I don't know of an ASP.NET function to “really URL-encode” strings for use in a path, so all I can recommend is doing a string replace of + to %20 after encoding.

Alternatively, avoid using spaces in path parts, eg. by replacing them with -. It's common to ‘slug’ titles being inserted to URLs, reducing them to simple alphanumerics and ‘safe’ punctuation, to avoid filling the URL with ugly %nn sequences.

This is an IIS security setting. There is a standard request filter that rejects URLs containing + (plus) characters.

You can disable it for your web, adding this to your web.config:

<configuration>
   ...
   <system.webServer>
      ...
      <security>
          <requestFiltering allowDoubleEscaping="true" />
      </security>
    </system.webServer>
    ...
</configuration>