MVC C# DropDownListFor 具有前一个 select 的值

Question

C# fiddle screenshot

https://dotnetfiddle.net/MadhI5

型号：

public class IndexViewModel
{
    public List<SelectListItem> Sel { get; set; }
    public string Val1 { get; set; }
    public string Val2 { get; set; }
    public string Val3 { get; set; }
    public string Val4 { get; set; }
    public string Val5 { get; set; }
}

控制器：

public ActionResult Index()
{
    var vm = new IndexViewModel();

    vm.Sel = new List<SelectListItem>();
    vm.Sel.Add(new SelectListItem() { Text = "Opt1", Value = "1" });
    vm.Sel.Add(new SelectListItem() { Text = "Opt2", Value = "2" });
    vm.Sel.Add(new SelectListItem() { Text = "Opt3", Value = "3" });

    vm.Val1 = "3";
    vm.Val2 = null;
    vm.Val3 = "1";
    vm.Val4 = null;
    vm.Val5 = "3";

    return View(vm);
}

查看：

@model HelloWorldMvcApp.IndexViewModel
@{
    Layout = null;
}

@Html.DropDownListFor(m => m.Val1, Model.Sel, "-")
@Html.DropDownListFor(m => m.Val2, Model.Sel, "-")
@Html.DropDownListFor(m => m.Val3, Model.Sel, "-")
@Html.DropDownListFor(m => m.Val4, Model.Sel, "-")
@Html.DropDownListFor(m => m.Val5, Model.Sel, "-")

所选值为

• Val1: 3 (正确)
• Val2: 3（应为“-”）
• Val3: 1 (正确)
• Val4: 1（应为“-”）
• Val5: 3 (正确)

查看 fiddle 演示。为什么 Val2 和 Val4 在 View 中不为空？这是一个框架错误？

Answer 1

...
vm.Val1 = "3";
vm.Val2 = "";
vm.Val3 = "1";
vm.Val4 = "";
vm.Val5 = "3";
...

使用 "" 代替 null 有效