Routing in MVC 6

In your original webapi code, you were using Routes.MapHttpRoute which adds webapi specific routes. This is different from an MVC route which won´t take into account the parameters in the action, for instance you would have the same problem in MVC 5 if you were using Routes.MapRoute.

The same thing is happening in your MVC 6 code, since you are adding a standard MVC route using routes.MapRoute. In both cases the framework is finding 2 controller actions matching the same route with no additional constraints. It needs some help in order to select one of those 2 actions.

The easiest way to disambiguate the api actions would be using attribute routing instead of defining a route, as in this example:

[Route("v1/[controller]")]
public class UsersController : Controller
{
    [HttpGet("{id:int}")]
    public IActionResult Users(long id)
    {
        return Json(new { name = "Example User" });
    }

    public IActionResult Users()
    {
        return Json(new { list = new[] { "a", "b" } });
    }
}

There are other options that would let you change the behaviour of the MVC routing in MVC 6. You could create your own IActionConstraint attribute to enforce having or not a given parameter. That way one of those actions requires an id parameter in the route while the other requires not to have an id parameter (Warning, untested code):

public class UsersController : Controller
{
    [RouteParameterConstraint("id", ShouldAppear=true)]
    public IActionResult Users(long id)
    {
        return Json(new { name = "Example User" });
    }

    [RouteParameterConstraint("id", ShouldNotAppear=true)]
    public IActionResult Users()
    {
        return Json(new { list = new[] { "a", "b" } });
    }
}

public class RouteParameterConstraintAttribute : Attribute, IActionConstraint
{
    private routeParameterName;

    public RouteParameterConstraintAttribute(string routeParameterName)
    {
        this.routerParamterName = routerParameterName;
    }

    public int Order => 0;
    public bool ShouldAppear {get; set;}
    public bool ShouldNotAppear {get; set;}

    public bool Accept(ActionConstraintContext context)
    {
        if(ShouldAppear) return context.RouteContext.RouteData.Values["country"] != null;
        if(ShouldNotAppear) return context.RouteContext.RouteData.Values["country"] == null;

        return true;
    }
}

A better option to deal with webapi 2 style controllers would be adding conventions into the MVC pipeline. This is exactly what the Microsoft.AspNet.Mvc.WebApiCompatShim is doing to help migrating webapi 2 controllers. You can see the conventions added here. Check this guide for a quick overview of this package.