如何在mvc中的HttpPost方法之后保留下拉選擇的值？

我正在加載下拉值資料庫。

我的觀點 DropdownDisplay.cshtml

@model RecommendModel                                           
<form asp-controller="Sample" asp-action="MyPostAction" method="post">
@if (ViewBag.ddl2 != null)
{
@Html.DropDownListFor(m => m.ID, ViewBag.ddl2 as SelectList, "Select value", new { @class = "form-control", @required = "required" })
 }
<button id="btnSubmit" class="btn btn-pink btn-sm">Submit</button>
   </form>

我的控制器推薦.cs

[HttpGet]
    public ActionResult DropdownDisplay(string eID)
    {
        List<RecommendModel> dropdownlist = new List<RecommendModel>();
        dropdownlist = gateway.SelectDomainByCompanyID(CompanyID);
        ViewBag.ddl2= new SelectList(dropdownlist , "ID", "Name");  
        return View();
    }
    
    
    [HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult MyPostAction(RecommendModel recommendModel)
    {
        
        //posting data and redirecting to same view 
        
        return RedirectToAction("DropdownDisplay", "Recommendation");
        
    }

如何在發布方法后保留選定的值并重定向到相同的視圖。您也可以建議另一種方法。

uj5u.com熱心網友回復：

使用 JavaScript 中的 __doPostBack() 函式傳遞您的值，因為您需要在 __doPostBack 函式中傳遞 2 個引數 1- EVENTTARGET （包含被單擊的控制元件） 2- EVENTARGUMENT （您要保存的任何值）

所以像這樣使用它

__doPostBack("<%=document.getElementById('yourcontrolid.ClientId')%>","your parameters that you want to save");

但是要讓 __doPostBack 作業，您需要在 html 檔案中使用腳本管理器，因此在此處插入腳本管理器，然后設定 OnClientClick="yourJavaScriptFunction();" 在您的下拉串列控制元件上，然后在您從下拉串列中選擇一個值時將 AutoPostBack 設定為 true，以便它可以重新加載頁面，但是當您這樣做時，您必須在包含您的 doPostBack 函式的腳本標記內初始化 yourJavaScriptFunction()

IE

<asp:DropDownList runat="server" Id="dropdownlist1" ClientIdMode="static" OnClientClick="yourJavaScriptFunction();" OnClick="DropDownList_SelectedIndexChange" >

<script>


function yourJavaScriptFunction() {
__doPostBack("<%=document.getElementById('dropdownlist1.ClientId')%>","your parameters that you want to save");}

對于 aspx.cs 檔案上的代碼，創建一個捕獲回發的事件函式

    public void DropDownList1_SelectedIndexChange(object sender, EventArgs e)
    {
    //yourcode
    }

uj5u.com熱心網友回復：

如果對視圖資料模型應用一些更改會更容易。

使用以下視圖模型：

public class ViewModel
{
    public IList<SelectListItem> SelectedCompanies { get; set; }
    public int? SelectedCompanyId { get; set; }
}

視圖資料模型的這種結構將提供保留選定項的能力，將其傳遞給控制器??并在下次渲染視圖時將其用作選定項。

在控制器中準備資料模型并將其傳遞給強型別視圖：

[HttpGet]
public ActionResult DropDownDisplay(string eID)
{
    var dropdownlist = new List<RecommendModel>();
    dropdownlist = gateway.SelectDomainByCompanyID(CompanyID);
    //dropdownlist.Insert(0, new RecommendModel { ID = 0, Name = "Select value" });    

    var vm = new ViewModel()
    {
        SelectedCompanies = new SelectList(dropdownlist, "ID", "Name").ToList()    
    };
    //vm.SelectedCompanyId = 2; // Set default value if it's necessary
    return View(vm);
}

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult MyPostAction(ViewModel vm)
{
    if (ModelState.IsValid)
    {
        // Processing data ...
    }
    return View("DropDownDisplay", vm);
}

在視圖中：

@model Models.ViewModel

<form asp-controller="Sample" asp-action="MyPostAction" method="post">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    @*
        This `for` loop is used to prepare a hidden context for the MVC binding to be used properly on the controller side.
        The `SelectedCompanies` values will be posted to controller and used when redirected to the
        view the next time.
        An another approach is to pass only the selected value and use it to prepare `SelectedCompanies` list
        in the same way as it created the first time.
    *@

    @for (int i = 0; i < Model.SelectedCompanies.Count(); i  )
    {
        @Html.Hidden("SelectedCompanies["   i   "].Text", Model.SelectedCompanies[i].Text)
        @Html.Hidden("SelectedCompanies["   i   "].Value", Model.SelectedCompanies[i].Value)
    }

    <div class="col-md-10">
        @Html.DropDownListFor(model => model.SelectedCompanyId, Model.SelectedCompanies, "Select a value", new { @class = "form-control", @required = "required" })
    </div>
    <br>
    <input type="submit" value="Submit" class="btn btn-pink btn-sm" />
</form>

uj5u.com熱心網友回復：

 // Summary:
        //     Initialize a new instance of Microsoft.AspNetCore.Mvc.Rendering.SelectList.
        //
        // Parameters:
        //   items:
        //     The items.
        //
        //   dataValueField:
        //     The data value field.
        //
        //   dataTextField:
        //     The data text field.
        //
        //   selectedValue:
        //     The selected value.
        public SelectList(IEnumerable items, string dataValueField, string dataTextField, object selectedValue)
        {
            throw null;
        }

你可以發現SelectLIst類有一個可以傳遞選定值的多載，所以你只需要更改你的 post 方法中的代碼，如：

[HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult MyPostAction(RecommendModel recommendModel)
    {
        
        //posting data and redirecting to same view 

        List<RecommendModel> dropdownlist = new List<RecommendModel>();
        dropdownlist = gateway.SelectDomainByCompanyID(CompanyID);
        ViewBag.ddl2= new SelectList(dropdownlist , "ID", "Name",recommendModel.ID);
        
        //Here just return `View` instead of `RedirectToAction`, otherwise you will get in get method agagin.
        return View("DropdownDisplay");
        
    }

演示：

標籤：asp.net-mvc asp.net 核心 模型视图控制器

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