在 ASP.NET Core MVC 中提交后，我的级联下拉列表丢失了选定的值？

Question

How can I keep selected values for both dropdown after submit action?

In my scenarios, my cascaded dropdown is populating from partial view. I'm new to ASP.NET Core MVC. Let me know if you want more clarifications.

My view:

<form asp-controller="Recommendation" asp-action="SubmitData" method="post">
    <select id="States" class="form-control selectpicker" asp-for="StateID" asp- 
            items="@(new SelectList(ViewBag.StateList,"StateID","State"))"
            placeholder="Select Categories"                                          
                 onchange="console.log($(this).children(':selected').length)">
    </select>              
    @Html.DropDownListFor(m => m.CityID, new SelectList(""), new {@class="select2 
                  form-control", @style = "width: 100%" })            
    <button id="btnSubmit" class="btn btn-secondary btn-sm">Submit</button>           
 </form>

onChange function on first dropdown to call 2nd one:

<script type="text/javascript">
            $(document).ready(function () {
                $("#States").change(function () {
                    var StateID = $(this).val();
                    /*debugger;*/
                    $("#CityID").empty();
                    $.ajax({
                        type: "Get",
                        url: "/Recommendation/GetCityList?iStateID=" + StateID,  
                        contentType: "html",
                        success: function (response) {
                            $("#CityID").append(response);                     
                        },
                        error: function (jqXHR, textStatus, errorThrown) {
                        }
                    })
                })
            });
</script>

Partial View for Child dropdown

<option value="">Select City</option>
        @if (ViewBag.CityOptions != null)
        {
            foreach(var item in ViewBag.CityOptions)                        
            {
                <option value="@item.Value">@item.Text</option>
            }
        }

Controller:

[HttpGet]
public ActionResult IndexGet()         
{   // From where I get  values.
        Entity entity = new Entity();
        StateList = gateway.SelectList();
        StateList.Insert(0, new Model { StateID = 0, State = "Select State" });          
        ViewBag.StateList = StateList;                
        return View();
}

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult SubmitData(RecommendModel recommendModel)       
{  // Submit form method and I used  RedirectToAction for calling view again.
        {            
        }            

        return RedirectToAction("IndexGet", "Recommendation");
}

[HttpGet]
public ActionResult GetCityList(long iStateID)     
{  // For partial call
        Entity entity = new Entity();
        MCAlist = entity.GetCityList(iStateID);
        ViewBag.CityOptions = new SelectList(MCAlist,"MCAID","MCA");      
        return PartialView("_CityOptionPartial");
} 

Answer 1

You can send same model again in post action.

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult IndexGet(RecommendModel recommendModel)       
{                        
     return View(model);
}