从强类型视图回发DropDownList

本文介绍了从强类型视图回发DropDownList的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

这与先前的问题有关从数据库绑定下拉列表有关.下拉列表被绑定并填充，但是抛出未设置为对象实例的对象引用".提交表单时浏览器出现错误.

This is related to an earlier question on binding a dropdownlist from a database. The dropdownlist is being bound and populated but is throwing an "Object reference not set to an instance of an object" error in the browser when submitting the form.

调试时，我在m =>上添加了一个快速监视.Html.DropDownListFor(...)帮助程序方法的m.SelectedDepartment参数，并得到:因为它不是委托类型，所以无法将lambda表达式转换为类型'object'".

When debugging, I added a quick watch on the m => m.SelectedDepartment parameter of the Html.DropDownListFor(...) helper method and got: "Cannot convert lambda expression to type 'object' because it is not a delegate type".

查看:

@model BudgetDemo.Models.BudgetsActualsViewModel @using (Html.BeginForm("GetBudgetsActuals", "BudgetsActuals", FormMethod.Post)) { @Html.DropDownListFor(m => m.SelectedDepartment, Model.Departments, "Select Department", new { @class = "form-control" }) }

ViewModel:

public class BudgetsActualsViewModel { [Display(Name = "Cost Center/Department")] [Required(ErrorMessage = "Cost Center/Department is required.")] [StringLength(62)] public string SelectedDepartment { get; set; } public List<SelectListItem> Departments { get; set; } }

控制器:

// GET: Render view with dropdowns public ActionResult GetBudgetsActuals() { repo = new BudgetDemoRepository(); ModelState.Clear(); try { return View(repo.GetBudgetsActuals()); } catch { return View("Error"); } }

存储库:

public BudgetsActualsViewModel GetBudgetsActuals() { ... BudgetsActualsViewModel budgetsActuals = new BudgetsActualsViewModel() { Departments = new List<SelectListItem>() }; // Query returning correct data from DB here for (int i = 0; i < ds.Tables[0].Rows.Count; i++) { budgetsActuals.Departments.Add( new SelectListItem { Text = ds.Tables[0].Rows[i]["Department"].ToString(), Value = ds.Tables[0].Rows[i]["ListValue"].ToString() } ); } return budgetsActuals; }

更新

现在正在工作.更新/添加了以下代码，以便在回发后显示包含所选部门值的成功消息来重新呈现表单.

It's working now. The following code was updated/added to get the form re-rendering after a postback with a success message containing the value of the selected department.

查看:

@model BudgetDemo.Models.BudgetsActualsViewModel @Html.AntiForgeryToken() @using (Html.BeginForm("GetBudgetsActuals", "BudgetsActuals", FormMethod.Post)) { @Html.ValidationSummary(true, "", new { @class = "text-danger" }) @if (TempData["SuccessMessage"] != null) { <p class="alert alert-success" id="successMessage">@TempData["SuccessMessage"]</p> } @Html.DropDownListFor(m => m.SelectedDepartment, Model.Departments, "Select Department", new { @class = "form-control" }) @Html.ValidationMessageFor(model => model.SelectedDepartment, "", new { @class = "text-danger" }) }

控制器:

// GET public ActionResult GetBudgetsActuals() { Populate Department dropdown from DB and Year dropdown (static) here repo = new BudgetDemoRepository(); //ModelState.Clear(); try { return View(repo.GetBudgetsActuals()); } catch { return View("Error"); } } // POST [HttpPost] public ActionResult GetBudgetsActuals(BudgetsActualsViewModel model) { try { repo = new BudgetDemoRepository(); if (ModelState.IsValid) { TempData["SuccessMessage"] = "Value of SelectedDepartment is: " + model.SelectedDepartment; return View(repo.GetBudgetsActuals(model)); } else { model.Departments = repo.GetBudgetsActuals().Departments; return View(model); } } catch { return View("Error"); } }

存储库-处理POST的新的重载方法

Repository - New overloaded method to handle POST

public BudgetsActualsViewModel GetBudgetsActuals(BudgetsActualsViewModel model) { .... // Get the data again .... budgetsActuals.SelectedDepartment = model.SelectedDepartment; for (int i = 0; i < ds.Tables[0].Rows.Count; i++) { budgetsActuals.Departments.Add( new SelectListItem { Text = ds.Tables[0].Rows[i]["Department"].ToString(), Value = ds.Tables[0].Rows[i]["ListValue"].ToString() } ); } return budgetsActuals; }

推荐答案

修改 controller ，使其具有支持 POST 调用的新api.请执行以下操作.

Modify the controller to have a new api that supports a POST call. Do as follows.

向此方法添加一个get方法，该方法将在加载视图时被调用，如下所示:

Add this method a get method that is called when the view is loaded, like this:

[HttpGet] public ActionResult GetBudgetsActuals() { repo = new BudgetDemoRepository(); ModelState.Clear(); try { return View(repo.GetBudgetsActuals()); } catch { return View("Error"); } }

添加在提交表单时调用的post方法:

Add a post method that is called when the form is submitted:

[HttpPost] // Decorate with this attribute public ActionResult PostBudgetsActuals(BudgetDemo.Models.BudgetsActualsViewModel model) { var selectedDepartment = model.SelectedDepartment; // dostuff with the model return somethingFromHere; }

并像这样更改视图:

@model BudgetDemo.Models.BudgetsActualsViewModel @using (Html.BeginForm("PostBudgetsActuals", "BudgetsActuals", FormMethod.Post)) { @Html.DropDownListFor(m => m.SelectedDepartment, Model.Departments, "Select Department", new { @class = "form-control" }) }

如果您遇到 DropDownListFor 方法的重载问题，那么您也可以尝试使用 DropDownList 方法，如下所示:

---

If you are getting an issue with the overload of DropDownListFor method, then you can also try DropDownList method like below:

@using (Html.BeginForm("PostBudgetsActuals", "BudgetsActuals", FormMethod.Post)) { @Html.DropDownList("SelectedDepartment", Model.Departments, "Select Department", new { @class = "form-control" }) }

更新

// POST [HttpPost] public ActionResult GetBudgetsActuals(BudgetsActualsViewModel model) { try { repo = new BudgetDemoRepository(); if (model.SelectedDepartment != null) { TempData["SuccessMessage"] = "Value of SelectedDepartment is: " + model.SelectedDepartment; } model.Departments = repo.GetBudgetsActuals().Departments; return View(model); } catch { return View("Error"); } }