Question

我正在尝试从下拉列表中清除选定的值，但该值仍然存在。使用@Html.DropDownListFor

是相同的行为

控制器

public class HomeController : Controller
{

    [Route("/Home/Index")]
    [Route("/Home/Index/{Category}")]
    [Route("/Home/Index/{Category}/{Type}")]
    public IActionResult Index(HomeModel model)
    {
        // Issue is here
        // for url: home/index/accessories/test
        // "Category" is cleared if it is not valid "type"
        // but still "Accessories" remains selected in the drop down
        if (model.Type != "Electronics" && model.Type != "Furniture")
        {
            model.Category = string.Empty;
        }

        return View(new HomeModel() { Category = model.Category, Type = model.Type });
    }

查看

@model WebApplication1.Controllers.HomeModel

<select asp-for="Category" asp-items="@Model.Categories"></select>
<select asp-for="Type" asp-items="@Model.Types"></select>

模型

public class HomeModel
{
    public string Category { get; set; }

    public string Type { get; set; }

    public List<SelectListItem> Categories { get; set; } = new List<SelectListItem>
    {
        new SelectListItem() { Text="Computers & Laptops", Value="Computers-Laptops" },
        new SelectListItem() { Text="Accessories", Value="Accessories" },
    };

    public List<SelectListItem> Types { get; set; } = new List<SelectListItem>
    {
        new SelectListItem() { Text="Electronics", Value="Electronics" },
        new SelectListItem() { Text="Furniture", Value="Furniture" },
    };
}

更新

我试图在“类别”下拉列表中添加一个空值，但仍然没有运气。

 <select asp-for="Category" asp-items="@Model.Categories">
        <option value="">Select Category</option>
 </select>

Answer 1

问题出在ModelState之内。发生的情况是，当将URL参数绑定到视图模型时，会将值添加到模型状态字典中。渲染视图时，不仅将HomeModel传递到视图，而且在场景下也传递ModelState-它具有比视图模型更高的priority。因此，当您重置model.Category时，Category中的ModelState键仍然具有值accessories，这就是为什么在下拉列表中选择它的原因。

解决方案很简单，调用.Clear()清除模型状态并使视图模型获胜：

[Route("/Home/Index")]
[Route("/Home/Index/{Category}")]
[Route("/Home/Index/{Category}/{Type}")]
public IActionResult Index(HomeModel model) {
    // Issue is here
    // for url: home/index/accessories/test
    // "Category" is cleared if it is not valid "type"
    // but still "Accessories" remains selected in the drop down
    if (model.Type != "Electronics" && model.Type != "Furniture") {
        model.Category = string.Empty;
    }
    ModelState.Clear();
    return View(new HomeModel() { Category = model.Category, Type = model.Type });
}

下拉选择未清除

更新

1 个答案: