Funny MVC Routing Quirk

I recently ran into a funny quirk with ASP.NET MVC’s routing. I have a method to retrieve attachments which have been uploaded for orders in an ordering system. Originally, I had the method and the route defined like this:

Method signature:

public ActionResult AttachmentView(int orderId, string filename)

Route definition:

            routes.MapRoute(name: "Orders_AttachmentView",

                url: "Orders/AttachmentView/{orderId}/{filename}",

                defaults: new { controller = "Orders", action = "AttachmentView" }

            );

When I tried to call this method, I’d get an error from IIS saying that the resource wasn’t found. I checked and double-checked the route: no problems, everything was correct. I put a breakpoint in the method to ensure that the code wasn’t being reached, and it wasn’t.

Then I thought that maybe the extension on the filename parameter was throwing the route off. I changed the method signature and route definition to reverse the two parameters so that the filename wouldn’t be the last parameter:

Method signature:

public ActionResult AttachmentView(string filename, int orderId)

Route definition:

            routes.MapRoute(name: "Orders_AttachmentView",

                url: "Orders/AttachmentView/{filename}/{orderId}",

                defaults: new { controller = "Orders", action = "AttachmentView" }

            );

And that did the trick. I guess having that URL end with something like “.gif” was throwing the routing off, which makes sense in hindsight.