TempData is not being used in HttpPost Method

Question

I keep receiving "NullReferenceException" error when i click button in my view. I created a list of the cities class and keep the data in TempData["cities"]. When i click the button, i want to redirect the page to the city name view and keep that city instance in a new TempData but the code doesnt even go in to the foreach loop. Can you help me?

[The photo of my code is here.] [1]: https://i.stack.imgur.com/zopfc.png

public IActionResult CitiesOnMap()
{
   List<City> cities = new List<City>();

   cities.Add(new City { CityName = "Hatay", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "Konya", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "İstanbul", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "İzmir", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "Muğla", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "Antalya", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "Bursa", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "Manisa", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "Mersin", Latitude = 123, Longtitude = 435 });
   cities.Add(new City { CityName = "Ankara", Latitude = 123, Longtitude = 435 });
  
   TempData["cities"] = cities;
   return View(); 
}

[HttpPost]
public IActionResult CitiesOnMap(string SelectedCity)
{    
    foreach (var redirectedCity in TempData["cities"] as List<MVC.Models.City>)
     {
        Console.WriteLine("bumbum");
         if (String.Equals(redirectedCity.CityName, SelectedCity))
         {             
             TempData["selectedCity"] = redirectedCity;
             return RedirectToAction(redirectedCity.CityName , "City");
         }
     }    
    return RedirectToAction(SelectedCity, "City"); ;
}

.cshtml view:

@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@{
    ViewData["Title"] = "CitiesOnMap";
}


<div class="text-center pt-5">
    <h1 class="display-4 pb-2 ">LET'S SEE THE CITIES!</h1>
    <p class="pb-3">Now, Please Choose A City You Want to See on Map.</p>
    <h3> @TempData["SelectedMap"] </h3>

    <div class="text-center mx-auto pt-5">
        <form asp-action="CitiesOnMap" asp-controller="City" method="post">

            <div class="row">
                <div class="col-9 mx-auto">
                    <select id="drpEmpList" name="SelectedCity" class="form-control">
                        <option disabled selected hidden>Choose a City </option>

                        @foreach (var name in TempData["cities"] as List<MVC.Models.City>)
                        {
                            <option>@name.CityName </option>
                        }

                    </select>
                    @{ TempData.Keep("cities"); }
                    <input type="hidden" />

                </div>
            </div>
            <div class="row">
                <div class="col-md-12 pt-4">
                    
                    <input type="submit" name="btnSubmit" value="Let's See" class="btn btn-success btn-lg" />
                </div>
            </div>

        </form>
    </div>
</div>

And now I'm getting HTTP ERROR 500

Answer 1

Are you reading the tempdata in you view html page? if yes, it always returns null for the next request. So you can use TempData.Keep("cities") after reading the data. so it will available for next requests

Answer 2

First you should check if TempData["cities"] is null.

if(TempData["cities"] != null)
{
foreach (var redirectedCity in TempData["cities"] as List<MVC.Models.City>)
     {
        Console.WriteLine("bumbum");
         if (String.Equals(redirectedCity.CityName, SelectedCity))
         {
             
             TempData["selectedCity"] = redirectedCity;
             return RedirectToAction(redirectedCity.CityName , "City");
         }


     } 
} 

and you have to be sure about TempData["cities"] type is List<MVC.Models.City>.

Answer 3

Your tempdata always null.

public IActionResult CitiesOnMap(string SelectedCity)
    {
        List<City> cities = new List<City>();

        cities.Add(new City { CityName = "Hatay", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "Konya", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "İstanbul", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "İzmir", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "Muğla", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "Antalya", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "Bursa", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "Manisa", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "Mersin", Latitude = 123, Longtitude = 435 });
        cities.Add(new City { CityName = "Ankara", Latitude = 123, Longtitude = 435 });

        TempData["cities"] = cities;

        var res = TempData["cities"];
        foreach (var redirectedCity in TempData["cities"] as List<City>)
        {
            Console.WriteLine("bumbum");
            if (String.Equals(redirectedCity.CityName, SelectedCity))
            {
                TempData["selectedCity"] = redirectedCity;
                return RedirectToAction(redirectedCity.CityName, "City");
            }
        }
        return RedirectToAction(SelectedCity, "City"); ;
    }