MVC DropDownListFor空引用[复制]

Question

这个问题已经在这里有一个答案：

• What is a NullReferenceException, and how do I fix it? 31个回答

该模型用于定义视图：

namespace OnlineStore.ViewModels
{
    public class SubCategoryVM
    {
        [Key]
        public int ID { get; set; }
        [Required]
        public virtual string Name { get; set; }
        [Required(ErrorMessage = "Parent Category Name is required")]
        public virtual string ParentName { get; set; }
        public IEnumerable<SelectListItem> categoryNames { get; set; }
    }
}

内部控制：

public ActionResult createSubCategory()
{
    SubCategoryVM model = new SubCategoryVM();
    var cNames = db.Categories.ToList();
    model.categoryNames = cNames.Select(x
        => new SelectListItem
        {
            Value = x.Name,
            Text = x.Name
        });
    return View(model);
}
[HttpPost]
public ActionResult createSubCategory(int? id, SubCategoryVM model)
{
    SubCategory sc = new SubCategory();
    if (ModelState.IsValid)
    {
        sc.ParentName = model.ParentName;
        sc.Name = model.Name;
    }
    return View();
}

并查看：

@model OnlineStore.ViewModels.SubCategoryVM        

<div class="form-group">
            @Html.LabelFor(model => model.ParentName, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.ParentName, Model.categoryNames, "--Please select an option--", new { @class = "form-control" })
                @Html.ValidationMessageFor(model => model.ParentName, "", new { @class = "text-danger" })
        </div>

这个代码就行抛出一个空引用异常@Html.DropDownListFor(model => model.ParentName, Model.categoryNames, "--Please select an option--", new { @class = "form-control" })说：

Model.categoryName（对象引用不设置为一个对象的一个​​实例）。

请帮我调试。

提前致谢。

Answer 1

问题是，当您发布形式和形式无效的情况下，返回与表单数据的视图，categoryNames模型变得无效，你必须再次返回与模型视图之前重新填充categoryNames。

因此，如下更新createSubCategory POST方法：

[HttpPost]
public ActionResult createSubCategory(int? id, SubCategoryVM model)
{
    SubCategory sc = new SubCategory();
    if (ModelState.IsValid)
    {
        sc.ParentName = model.ParentName;
        sc.Name = model.Name;
    }

    var cNames = db.Categories.ToList();
    model.categoryNames = cNames.Select(x
        => new SelectListItem
        {
            Value = x.Name,
            Text = x.Name
        });

    return View(model);
}