Why does Html.BeginForm generate empty action?

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 &amp; Export" />
}

The generated HTML for this form has a blank action:

<form action="" method="post">  <input type="submit" value="Complete &amp; 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.Op开发者_StackOverflowtional, actionId = UrlParameter.Optional }
        );

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

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.