C# ArgumentNullException:值不能为null。参数名称：items selectlist.net core

Selectlist对于

Get

请求非常有效。但是对于

Post

请求，它会给出

null

异常

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

控制器代码：

var category = _context.CategoryTbl.ToList();
List<SelectListItem> li = new List<SelectListItem>();
li.Add(new SelectListItem { Text = "Select Category", Value = "0" });
foreach (var m in category)
{
     li.Add(new SelectListItem { Text = m.Name, Value = m.Id.ToString() });
     ViewBag.category =li;
}

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

我发现以下错误：

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

ArgumentNullException:值不能为null。参数名称：items

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

三件事：

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

将选项存储为视图模型上的属性更合适。这就解决了类似于动力学的问题。当您保持强类型时，追踪问题会容易得多

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

您不需要创建

选择列表

。所需要的只是一个您已经拥有的

IEnumerable

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

最好在此处使用

SelectTagHelper

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

综上所述，在您的模型上添加：

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

public IEnumerable<SelectListItem> CategoryOptions { get; set; }

public IEnumerable CategoryOptions{get；set；}

那么在你看来,

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

<select asp-for="Category" asp-items="@Model.CategoryOptions"></select>

三件事：

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

将选项存储为视图模型上的属性更合适。这就解决了类似于动力学的问题。当您保持强类型时，追踪问题会容易得多

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

您不需要创建

选择列表

。所需要的只是一个您已经拥有的

IEnumerable

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

最好在此处使用

SelectTagHelper

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

综上所述，在您的模型上添加：

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

public IEnumerable<SelectListItem> CategoryOptions { get; set; }

public IEnumerable CategoryOptions{get；set；}

那么在你看来,

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

<select asp-for="Category" asp-items="@Model.CategoryOptions"></select>

---

它与您的

POST

操作相关，如果您返回相同的视图，您还需要一个

ViewBag.category

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

您可以使用get方法中的方式在返回视图之前对其进行初始化，也可以简单地使用

newselectlist（\u context.CategoryTbl，“Id”，“Name”）

，请参阅以下代码：

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

[HttpPost]  
public async Task<IActionResult> MyPostMethodName(Model model)
    {
        if (!ModelState.IsValid)
        {

        }
        //your post logic
        //...
        //need a  ViewBag.category if you return the same view,
        ViewBag.category = new SelectList(_context.CategoryTbl, "Id", "Name", model.Category);

        return View(product);
    }

[HttpPost]
公共异步任务MyPostMethodName（模型）
{
如果（！ModelState.IsValid）
{
}
//你的后逻辑
//...
//如果返回相同的视图，则需要ViewBag.category，
ViewBag.category=new SelectList（_context.CategoryTbl，“Id”，“Name”，model.category）；
返回视图（产品）；
}

它与您的

POST

操作相关，如果您返回相同的视图，您还需要一个

ViewBag.category

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

您可以使用get方法中的方式在返回视图之前对其进行初始化，也可以简单地使用

newselectlist（\u context.CategoryTbl，“Id”，“Name”）

，请参阅以下代码：

@Html.DropDownListFor(model => model.Category, new SelectList(ViewBag.category, "Value", "Text"), new { @id = "ddlCategory", style = "width: 200px;", @class = "form-control input-lg" })

[HttpPost]  
public async Task<IActionResult> MyPostMethodName(Model model)
    {
        if (!ModelState.IsValid)
        {

        }
        //your post logic
        //...
        //need a  ViewBag.category if you return the same view,
        ViewBag.category = new SelectList(_context.CategoryTbl, "Id", "Name", model.Category);

        return View(product);
    }

[HttpPost]
公共异步任务MyPostMethodName（模型）
{
如果（！ModelState.IsValid）
{
}
//你的后逻辑
//...
//如果返回相同的视图，则需要ViewBag.category，
ViewBag.category=new SelectList（_context.CategoryTbl，“Id”，“Name”，model.category）；
返回视图（产品）；
}

您可以发布整个异常堆栈吗？不仅仅是错误消息。你能发布整个异常堆栈吗？不仅仅是错误信息。