Why does Html.BeginForm generate empty action? - asp.net-mvc-3

Question

I have a controller in an area called Admin
public class SiteVisitController : Controller
{
public ViewResult ReadyForCompletion() { ... }
public ViewResult CompleteAndExport() { ... }
}
and a view (ReadyForCompletion.cshtml) that has posts back to a different controller action on the same class
#using (Html.BeginForm( "CompleteAndExport", "SiteVisit" ))
{
<input type="submit" value="Complete & Export" />
}
The generated HTML for this form has a blank action:
<form action="" method="post"> <input type="submit" value="Complete & Export" />
</form>
I want to know why this has a blank action? For more info, I also added in a
#Url.RouteUrl(new { controller = "ReadyForCompletion", action = "SiteVisit", area = "Admin" })
which also printed out an empty string. Also, if I use an empty Html.BeginForm() it generates the correct action.
Registered routes are
context.MapRoute(
"Admin_manyParams",
"Admin/{controller}/{action}/{id}/{actionId}",
new { action = "Index", id = UrlParameter.Optional, actionId = UrlParameter.Optional }
);

Answer 1

I believe your problem is caused by having consecutive optional parameters. I was not able to replicate your problem until I changed the route to contain two optional parameters.
See: This article which explains the problem

Answer 2

For those of you encountering this issue using ASP.NET Core the root cause is the same, though the solution is slightly different. I first saw this in Core using multiple default values when calling .MapRoutes(). E.g.
routes.MapRoute(
name: "default",
template: "{controller}/{action}/{id?}",
defaults: new { controller = "Foo", action = "Bar" }
);
The workaround is to place the default values into the string template:
routes.MapRoute(
name: "default",
template: "{controller=Foo}/{action=Bar}/{id?}"
);
YMMV.