我觉得我有一个工作的想法，但是当我构建应用程序时，我得到一个“对象引用没有设置为对象的实例”错误。我只是想在选择状态后显示城市的下拉列表。我在一个单独的类中有一个状态列表，我用于表单中的下拉列表。我试图做的是获取所选状态并将其作为参数传递到另一个类中，以便选择要显示为另一个下拉列表的正确城市列表。因此，如果有人选择华盛顿州，那么西雅图和塔科马等城市就会出现，而不是来自不同州的城市。这是下降的剃刀（州工作正常）：
<div class="form-group">
@Html.LabelFor(model => model.State, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.DropDownListFor(model => model.State, WebGridProject.Models.StateCodes.GetStatesList(), new { @class = "form-control" })
@Html.ValidationMessageFor(model => model.State, "", new { @class = "text-danger" })
</div>
</div>
<div class="form-group">
@Html.LabelFor(model => model.City, htmlAttributes: new { @class = "control-label col-md-2" })
<div class="col-md-10">
@Html.DropDownListFor(model => model.City, WebGridProject.Models.CityNames.GetStateCities(Model.State), new { htmlAttributes = new { @class = "form-control" } })
@Html.ValidationMessageFor(model => model.City, "", new { @class = "text-danger" })
</div>
</div>
这是城市的课程：
public class CityNames
{
public static IEnumerable<SelectListItem> GetStateCities(string stateName)
{
IList<SelectListItem> cities = new List<SelectListItem>();
switch (stateName)
{
case "AK":
cities = new List<SelectListItem>
{
new SelectListItem() {Text = "", Value = ""},
new SelectListItem() {Text = "Anchorage", Value = "Anchorage"},
new SelectListItem() {Text = "Fairbanks", Value = "Fairbanks"},
};
break;
case "AL":
cities = new List<SelectListItem>
{
new SelectListItem() {Text = "", Value = ""},
new SelectListItem() {Text = "Auburn", Value = "Auburn"},
new SelectListItem() {Text = "Birmingham", Value = "Birmingham"},
new SelectListItem() {Text = "Dothan", Value = "Dothan"},
new SelectListItem() {Text = "Mobile", Value = "Mobile"},
};
break;
case "AZ":
cities = new List<SelectListItem>
{
new SelectListItem() {Text = "", Value = ""},
new SelectListItem() {Text = "Pheonix", Value = "Pheonix"},
new SelectListItem() {Text = "Flagstaff", Value = "Flagstaff"},
new SelectListItem() {Text = "Prescott", Value = "Prescott"},
new SelectListItem() {Text = "Tucson", Value = "Tucson"},
};
break;
case "AR":
cities = new List<SelectListItem>
{
new SelectListItem() {Text = "", Value = ""},
new SelectListItem() {Text = "Fayetteville", Value = "Fayetteville"},
new SelectListItem() {Text = "Fort smith", Value = "Fort smith"},
new SelectListItem() {Text = "Jonesboro", Value = "Jonesboro"},
new SelectListItem() {Text = "Texarkana", Value = "Texarkana"},
};
break;
default:
cities = new List<SelectListItem>
{
new SelectListItem() {Text = "", Value = ""},
new SelectListItem() {Text = "Please Select State", Value = "Please Select State"},
};
break;
}
return cities;
}
}
对我来说，错误似乎指向没有默认值并且在加载空值时存在问题。但是我在switch语句中有一个默认情况来解决这个问题。我是MVC的新手，我将继续自己研究这个问题。但是，如果有人有任何想法，我会全力以赴，因为我正在努力解决这个问题。仅供参考，州级也是IList＆lt;＆gt;返回状态名称列表。
要求发布行动：
public ActionResult MyHomePage()
{
return View();
}
//Post method
[HttpPost]
public ActionResult MyHomePage(WebGridTable TWG)
{
try
{
if (ModelState.IsValid)
{
UserManager UM = new UserManager();
UM.addNewUser(TWG);
ModelState.Clear();
return View();
}
return View();
}
catch (Exception e)
{
return View("Error");
}
}
行动并不意味着什么：
public void addNewUser(WebGridTable AddNew)
{
using (WebGridDBEntities4 db = new WebGridDBEntities4())
{
WebGridTable TWG = new WebGridTable();
TWG.Name = AddNew.Name;
TWG.DOB = AddNew.DOB;
TWG.AddressLine1 = AddNew.AddressLine1;
// TO DO: Figure out issue with accepting null value into database.
if (AddNew.AddressLine2 != null)
{
TWG.AddressLine2 = AddNew.AddressLine2;
}
else if (AddNew.AddressLine2 == null)
{
TWG.AddressLine2 = 0;
}
TWG.State = AddNew.State;
TWG.City = TWG.City;
TWG.Zip = AddNew.Zip;
TWG.Gender = AddNew.Gender;
db.WebGridTables.Add(TWG);
db.SaveChanges();
}
}
答案 0 :(得分：1)
您需要对States DropDownList的更改使用JQuery操作。将值发送到Controller中的特定Action，并调用States.GetStatesName（＆＃39; Value＆＃39;）。请查看下面的代码。我认为这会有所帮助。
$("#State").on('change',function () {
var loadingoption = $('<option></option>').text("Pleas Wait");
$('#city').attr("disabled","disabled").empty().append(loadingoption);
jQuery.getJSON("/Home/CityJson/" + $("#State > option:selected").attr("value"), function (data) {
var defaultoption = $('<option value="">Please choose a City</option>');
$('#city').removeAttr("disabled").empty().append(defaultoption);
jQuery.each(data, function (i) {
var option2 = $('<option></option>').attr("value", data[i].Name).text(data[i].Name);
$("#city").append(option2);
});
});
});
我使用了ActionResult＆＃34; CityJson＆＃34;在我的HomeController中。所以在getJSON中选择你的Action。
这是控制器：
public ActionResult CityJson(string id)
{
var state = db.States.FirstOrDefault(x => x.Name == id);
var model = db.Cities.Where(x => x.StateId == state.Id).Select(x => new { x.Name }).ToList();
return Json(model, JsonRequestBehavior.AllowGet);
}
你可以在Class中调用你的动作并使用Json发送它们。
祝你好运。