Dropdownlist

Question

我正在使用ViewBag填充下拉列表。在编辑页面中，我的Dropdown属性具有值，并且不能为空。但我遇到错误“可为空的对象必须具有值”。在“ @foreach（ViewBag.Designations中的可变项）”行中。我尝试在视图中使用HtmlHelper和普通循环。

//控制器

int orgId = Convert.ToInt32(Session["OrganisationId"]);
ViewBag.Designations = db.Designations.Where(r => r.OrganisationId ==orgId).ToList();

//查看

<select class="custom-select select2 select2-hidden-accessible" tabindex="-1" aria-hidden="true" id="DesignationId" name="DesignationId">
                                                    <option value="">Select Designation</option>
                                                    @foreach (var item in ViewBag.Designations)
                                                    {
                                                        if (item.DesignationId == Model.DesignationId)
                                                        {
                                                            <option value="@item.DesignationId" selected>@item.DesignationName</option>
                                                        }
                                                        else
                                                        {
                                                            <option value="@item.DesignationId">@item.DesignationName</option>
                                                        }

                                                    }
                                                </select>

//我也尝试过

@Html.DropDownListFor(model => model.DesignationId, new SelectList(ViewBag.Designations, "DesignationId", "DesignationName"), htmlAttributes: new { @class = "custom-select select2 select2-hidden-accessible", id = "DesignationId" })