邮政SelectList零

Question

我试图提交一个表单，将级联下拉列表和图像的结果发布回控制器，但在使用Html.DropDownList时收到以下错误

System.InvalidOperationException: 'There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'BreedDD'.' \

当我手动写出选择时，它会抛出一个NullReferenceException

根据一些研究表明，ViewBag将失去它的值，这将解释空引用。但我还没有找到对我有用的解决方案，而且我已经到了临界点。

FileUploadController.cs

using Microsoft.Ajax.Utilities;
using ShipleySwine;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace UploadingFilesUsingMVC.Controllers
{
    public class FileUploadController : Controller
    {
        private ShipleySwineContext db = new ShipleySwineContext();
        // GET: FileUpload    
        public ActionResult Index()
        {
            var breedlist = db.Boars.ToList();
            var list = breedlist.Distinct();
            var list2 = list.DistinctBy(e => e.Breed).ToList();
            List<SelectListItem> breedDDsli = new List<SelectListItem>();
            new SelectList(list2);
            foreach(var item in list2)
            {
                breedDDsli.Add(new SelectListItem
                {
                    Text= item.Breed,
                    Value = item.Breed
                });

                Debug.WriteLine(item.Breed);
            }
            ViewBag.breedDD = breedDDsli;

            return View();
        }
        [HttpPost]
        public ActionResult UploadFiles(string incomingbreed, string NameNoSpaces, HttpPostedFileBase file)
        {
            if (ModelState.IsValid)
            {
                try
                {


                    if (file != null)
                    {
                        string path = Path.Combine(Server.MapPath("~/UploadedFiles"), Path.GetFileName(file.FileName));
                        file.SaveAs(path);

                    }
                    ViewBag.FileStatus = "File uploaded successfully.";
                }
                catch (Exception)
                {

                    ViewBag.FileStatus = "Error while file uploading.";
                }

            }
            return View("Index");
        }

        public ActionResult getBoarsList(string boarbreed)
        {
            var breedlist = db.Boars.Where(breed => breed.Breed == boarbreed).ToList();
            var list = breedlist.Distinct();
            var list2 = list.DistinctBy(e => e.Name).ToList();
            List<SelectListItem> boarsDD = new List<SelectListItem>();
            new SelectList(list2);
            foreach (var item in list2)
            {
                boarsDD.Add(new SelectListItem
                {
                    Text = item.NameNoSpaces,
                    Value = item.NameNoSpaces
                });

                Debug.WriteLine(item.Breed);
            }
            ViewBag.BoarsDD = boarsDD;
            return PartialView("DisplayBoars");
        }

        //public ActionResult boarDetails(string boarName)
        //{
        //    var boarDetails = db.Boars.Where(name => name.Name == boarName);
        //    ViewBag.boarDetails = boarDetails;
        //    return(ViewBag.boar)
        //    //return PartialView("DisplayBoarDetails");
        //}
    }
}

Index.cshtml

@model ShipleySwine.Models.FileUpload
@{
                /**/


}

@using (Html.BeginForm("UploadFiles", "FileUpload", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.DropDownList("BreedDD", (IEnumerable<SelectListItem>)ViewBag.breedDD,"-- Select Breed --", new { @class = "form-control" })
            @*<select class = "form-control" id="BreedDD" name="breed">
                <option value="">-- Select Breed --</option>
                @foreach(var item in ViewBag.BreedDD)
                {
                    <option value="@item.Text">@item.Text</option>
                }
            </select>*@
            <select class="form-control" id="BoarsDD" name="NameNoSpaces">

            </select>
            <div class="col-md-10">
                @Html.EditorFor(model => model.file, new { htmlAttributes = new { @class = "form-control", @type = "file" } })
                @Html.ValidationMessageFor(model => model.file, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Upload" class="btn btn-primary" />
            </div>
        </div>
        <div class="form-group">
            <div class="col-md-offset-2 col-md-10 text-success">
                @ViewBag.FileStatus
            </div>
        </div>
    </div>
}

<script src="~/Scripts/jquery-3.3.1.min.js"></script>
<script>
    $(document).ready(function () {
        $("#BreedDD").change(function () {
            var breed = $(this).val();

            $.ajax({
                type: "post",
                url: "/FileUpload/getBoarsList?boarbreed=" + breed,
                contentType: "html",
                success: function (response) {

                    $("#BoarsDD").empty();
                    $("#BoarsDD").append(response);
                }
            })
        })

        //$("#BoarsDD").change(function () {
        //    var boar = $(this).val();

        //    $.ajax({
        //        type: "post",
        //        url: "/FileUpload/boarDetails?boarName=" + boar,
        //        contentType: "html",
        //        success: function (response) {
        //            console.log(response);
        //            console.log(response.val);
        //            $("#boarDetails").empty();
        //            $("#boarDetails").val(response);
        //        }
        //    })
        //})
    })
</script>

模型

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web;

namespace ShipleySwine.Models
{
    public class FileUpload
    {
        [DataType(DataType.Upload)]
        [Display(Name = "Upload File")]
        [Required(ErrorMessage = "Please choose file to upload.")]
        public string file { get; set; }
        public string breed { get; set; }
        public string NameNoSpace { get; set; }

    }
}

Answer 1

当表单在上传文件时回发时，ViewBag对象就会丢失。成功上传文件后，您将重定向回索引页。这一次，您的视图不接收DropdownList的DropdownList数据。因此，在成功上传ViewBag方法中的文件之后，需要为DropdownList设置UploadFiles，如下所示：

    [HttpPost]
    public ActionResult UploadFiles(string incomingbreed, string NameNoSpaces, HttpPostedFileBase file)
    {
        if (ModelState.IsValid)
        {
            try
            {


                if (file != null)
                {
                    string path = Path.Combine(Server.MapPath("~/UploadedFiles"), Path.GetFileName(file.FileName));
                    file.SaveAs(path);

                }
                ViewBag.FileStatus = "File uploaded successfully.";
            }
            catch (Exception)
            {

                ViewBag.FileStatus = "Error while file uploading.";
            }

        }

        //Add dropdown list to ViewBag
        var breedlist = db.Boars.ToList();
        var list = breedlist.Distinct();
        var list2 = list.DistinctBy(e => e.Breed).ToList();
        List<SelectListItem> breedDDsli = new List<SelectListItem>();
        new SelectList(list2);
        foreach(var item in list2)
        {
            breedDDsli.Add(new SelectListItem
            {
                Text= item.Breed,
                Value = item.Breed
            });

            Debug.WriteLine(item.Breed);
        }
        ViewBag.breedDD = breedDDsli;

        return View("Index");
    }