ReturnUrl in asp.net mvc is null for some reason

Question

public ActionResult LogOn(string returnUrl)
        {
            if (Request.IsAuthenticated)
            {
                return RedirectToAction(string.Empty, "home");
            }
            else
            {
                if (!string.IsNullOrWhiteSpace(returnUrl))
                {
               //http://localhost:666/en-us/account/logon?returnurl=%2fen-us%2fadminka
                     //..............
                }
                return View();

            }
        }

        [HttpPost]
        public ActionResult LogOn(LogOnViewModel model, string returnUrl)
        {
            if (ModelState.IsValid)
            {
                if (....)
                {
                    //..............
                    if (Url.IsLocalUrl(returnUrl) && returnUrl.Length > 1 && returnUrl.StartsWith("/")
                        && !returnUrl.StartsWith("//") && !returnUrl.StartsWith("/\\"))
                        return Redirect(returnUrl);
                    return RedirectToAction(string.Empty, "home");

                }
                else
                {
                    //..............
                }
            }

            return View(model);
        }

In HttpPost LogOn returnUrl is always equals null, even if it was not null in HttpGet LogOn.

Why? How do I fix it?

Answer 1

You need the returnUrl to be posted with the form post.

Probably the cleanest solution is to add returnUrl as a property to the LogOnViewModel:

    public class LogOnViewModel
{
    public string UserName { get; set; }
    public string Password { get; set; }
    public string ReturnUrl { get; set; }
}

Your get method would set that value:

[HttpGet]
public ActionResult LogOn(string returnUrl)
    {
       // code for already authenticated case left out for demo
            var model = new LogOnViewModel { ReturnUrl = returnUrl };
            return View(model);

        }
    }

In your form, you would persist that value as a hidden field:

@using (Html.BeginForm())
{
    @Html.HiddenFor(model => model.ReturnUrl)

    // rest of form code left out for demo purposes
}

Your post method would then have access to that value:

    [HttpPost]
    public ActionResult LogOn(LogOnViewModel model)
    {
        if (ModelState.IsValid)
        {
            if (....)
            {   string returnUrl = model.ReturnUrl;

                //..............
                if (Url.IsLocalUrl(returnUrl) && returnUrl.Length > 1 && returnUrl.StartsWith("/")
                    && !returnUrl.StartsWith("//") && !returnUrl.StartsWith("/\\"))
                    return Redirect(returnUrl);
                return RedirectToAction(string.Empty, "home");

            }
            else
            {
                //..............
            }
        }

        return View(model);
    }