基于跨多个下拉菜单的特定选定下拉菜单值的C# MVC标记文本区

基础概念

在C# MVC（Model-View-Controller）框架中，跨多个下拉菜单的特定选定值通常涉及到前端和后端的交互。前端负责展示下拉菜单和文本区，后端负责处理用户的选择并更新文本区的显示内容。

类型

静态绑定：下拉菜单的值在页面加载时就已经确定。
动态绑定：下拉菜单的值可以通过后端API动态获取。

应用场景

例如，在一个表单中，用户需要根据选择的国家和城市来获取该城市的详细信息并显示在文本区中。

示例代码

前端（View）

@model YourApplication.Models.FormModel

@using (Html.BeginForm("SubmitForm", "Home"))
{
    <div>
        @Html.DropDownListFor(model => model.Country, Model.Countries, "Select Country")
    </div>
    <div>
        @Html.DropDownListFor(model => model.City, Model.Cities, "Select City")
    </div>
    <div>
        @Html.TextAreaFor(model => model.Details)
    </div>
    <input type="submit" value="Submit" />
}

后端（Controller）

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new FormModel
        {
            Countries = new List<SelectListItem>
            {
                new SelectListItem { Text = "USA", Value = "USA" },
                new SelectListItem { Text = "Canada", Value = "Canada" }
            },
            Cities = new List<SelectListItem>
            {
                new SelectListItem { Text = "New York", Value = "NY" },
                new SelectListItem { Text = "Toronto", Value = "TO" }
            }
        };
        return View(model);
    }

    [HttpPost]
    public ActionResult SubmitForm(FormModel model)
    {
        if (ModelState.IsValid)
        {
            // 处理表单提交逻辑
            return RedirectToAction("Success");
        }
        return View(model);
    }
}

模型（Model）

public class FormModel
{
    public string Country { get; set; }
    public string City { get; set; }
    public string Details { get; set; }

    public List<SelectListItem> Countries { get; set; }
    public List<SelectListItem> Cities { get; set; }
}

遇到的问题及解决方法

问题：文本区没有根据下拉菜单的选择动态更新

原因：可能是前端JavaScript代码没有正确处理下拉菜单的变化事件，或者后端API没有正确返回数据。

解决方法：

前端JavaScript：

$(document).ready(function () {
    $('#Country').change(function () {
        var country = $(this).val();
        $.ajax({
            url: '@Url.Action("GetCities", "Home")',
            type: 'GET',
            data: { country: country },
            success: function (cities) {
                var citySelect = $('#City');
                citySelect.empty();
                $.each(cities, function (index, city) {
                    citySelect.append($('<option></option>').val(city.Value).text(city.Text));
                });
            }
        });
    });

    $('#City').change(function () {
        var city = $(this).val();
        $.ajax({
            url: '@Url.Action("GetDetails", "Home")',
            type: 'GET',
            data: { city: city },
            success: function (details) {
                $('#Details').val(details);
            }
        });
    });
});

后端API：

public ActionResult GetCities(string country)
{
    var cities = new List<SelectListItem>();
    if (country == "USA")
    {
        cities.Add(new SelectListItem { Text = "New York", Value = "NY" });
        cities.Add(new SelectListItem { Text = "Los Angeles", Value = "LA" });
    }
    else if (country == "Canada")
    {
        cities.Add(new SelectListItem { Text = "Toronto", Value = "TO" });
        cities.Add(new SelectListItem { Text = "Vancouver", Value = "VC" });
    }
    return Json(cities, JsonRequestBehavior.AllowGet);
}

public ActionResult GetDetails(string city)
{
    string details = "";
    switch (city)
    {
        case "NY":
            details = "New York is a bustling city with a rich history.";
            break;
        case "TO":
            details = "Toronto is known for its diversity and multiculturalism.";
            break;
        // 其他城市的情况
    }
    return Json(details, JsonRequestBehavior.AllowGet);
}