从强类型视图回发 DropDownList

Question

这与从数据库绑定下拉列表的有关。下拉列表正在绑定和填充，但在提交表单时在浏览器中抛出“未设置对象实例的对象引用”错误。

调试时，我在 Html.DropDownListFor(...) 辅助方法的 m => m.SelectedDepartment 参数上添加了一个快速观察并得到：“无法将 lambda 表达式转换为类型 'object' 因为它不是委托类型。

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

现在可以了。以下代码 updated/added 用于在回发后重新呈现表单，其中包含包含所选部门值的成功消息。

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");
    }
}

Repository - 处理 POST

的新重载方法

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;
}

Answer 1

修改 controller 以获得支持 POST 调用的新 api。如下操作。

将此方法添加到加载视图时调用的 get 方法，如下所示：

[HttpGet]
public ActionResult GetBudgetsActuals()
{
    repo = new BudgetDemoRepository();
    ModelState.Clear();

    try
    {                
        return View(repo.GetBudgetsActuals());
    }
    catch
    {
        return View("Error");
    }
}

添加一个 post 提交表单时调用的方法：

[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 方法重载的问题，那么您也可以尝试如下 DropDownList 方法：

@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");
     }
}

从强类型视图回发 DropDownList

Posting Back DropDownList from Strongly Typed View

c#

asp.net

model-view-controller

asp.net-mvc

asp.net-mvc-5