ASP.NET MVC: Mock controller.Url.Action

Here's how you could mock UrlHelper using MvcContrib's TestControllerBuilder:

var routes = new RouteCollection();
MvcApplication.RegisterRoutes(routes);
HomeController controller = CreateController<HomeController>();

controller.HttpContext.Response
    .Stub(x => x.ApplyAppPathModifier("/Home/About"))
    .Return("/Home/About");

controller.Url = new UrlHelper(
    new RequestContext(
        controller.HttpContext, new RouteData()
    ), 
    routes
);
var url = controller.Url.Action("About", "Home");
Assert.IsFalse(string.IsNullOrEmpty(url));

A cleaner way to do this is just use Moq(or any other framework you like) to Mock UrlHelper itself

var controller = new OrdersController();
var UrlHelperMock = new Mock<UrlHelper>();

controller.Url = UrlHelperMock.Object;

UrlHelperMock.Setup(x => x.Action("Action", "Controller", new {parem = "test"})).Returns("testUrl");

var url = controller.Url.Action("Action", "Controller", new {parem = "test"});
assert.areEqual("/Controller/Action/?parem=test",url);

clean and simple.