带有字典的 MVC 下拉列表答案

【问题标题】：MVC DropDownList with Dictionary带有字典的 MVC 下拉列表
【发布时间】：2017-01-06 22:42:45
【问题描述】：

我需要在视图上显示一个 DropDownList，其中包含 2 个不同表的值。

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获取编辑

var partners = db.Partners.Include(p => p.Company);
            var serviceProviders = db.ServiceProvider.Include(f => f.Building);
            ViewBag.Assigned = new Dictionary<string, string>();

            foreach (var p in partners)
            {
                ViewBag.Assigned.Add("P_" + p.PartnerID.ToString(), p.Name);
            }
            foreach (var f in serviceProviders)
            {
                ViewBag.Assigned.Add("FS_" + f.ServiceProviderID.ToString(), f.Name);
            }

此字典将包含如下所示的键值对：

“P_1”、“Abc”
“P_2”、“Def”
“SP_1”、“Ghi”
“SP_2”、“Jkl”
“SP_3”“Mno”

发布编辑

        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Edit([Bind(Include = "ReqID,State,Title,Description,Emission,Severity,BuildingID,Appointment,CUICreator,Photo1,Photo2,Photo3,Photo4,Photo5,IDPartner,IDServiceProvider,IDColaborator,Assigned")] Req req)
        {
            if (ModelState.IsValid)
            {
                db.Entry(req).State = EntityState.Modified;
                await db.SaveChangesAsync();
                return RedirectToAction("Index");
            }
            ViewBag.EdificioID = new SelectList(db.Buildings, "BuildingID", "Name", req.BuildingID);
            return View(req);
        }

在我的 Edit.cshtml 上，我尝试了：

@Html.DropDownListFor(model => model.Assigned, new SelectList(ViewBag.Assigned, "Key", "Value"), htmlAttributes: new { @class = "form-control" })

和

@Html.DropDownList("Atribuido", new SelectList(ViewBag.Atribuidos, "Key", "Value"))

他们都给出了同样的错误：

O valor não pode ser nulo.
Nome do parâmetro: items

Descrição: Exceção não processada ao executar o pedido Web atual. Consulte o rastreio da pilha para obter mais informações sobre o erro e o respetivo ponto de origem no código.

翻译成这样：

Value can not be null.
Parameter name: items

Description: Unhandled exception while executing the current Web request. See the stack trace for more information about the error and its source point in the code.

完成这项任务的最佳方法是什么？

【问题讨论】：

标签： dictionary razor html.dropdownlistfor asp.net-mvc-5 dropdownlistfor

【解决方案1】：

该错误消息与您从ArgumentNullException 获得的消息相匹配，当方法获得所需参数的空值时会抛出该消息。我怀疑错误来自SelectList constructor，因为它需要一个名为items 的参数。由于您没有在Post 处理程序中设置ViewBag.Assigned，因此当您尝试在发布后呈现编辑视图时，viewbag 项目可能会丢失并且为空。

【讨论】：