C# 具有键'；类别ID'；类型为'；System.Int32'；但必须为'型；IEnumerable<；选择列表项>'；

我试图将一些新帖子保存到我的数据库中，但我多次出错。。我在下面有一个“Posts.cs”类

在我的视图CreatePost.cshtml中，我有

@using (Html.BeginForm()) 
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.LabelFor(model => model.PostTitle, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.PostTitle, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.PostTitle, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.PostContent, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @*@Html.TextAreaFor(model => model.PostContent, new { htmlAttributes = new { @class = "form-control" } })*@
                <textarea class="form-control" name="PostContent" id="PostContent" rows="3"></textarea>
                @Html.ValidationMessageFor(model => model.PostContent, "", new { @class = "text-danger" })
            </div>
        </div>
                <div class="form-group">
            @Html.LabelFor(model => model.CategoryId, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.CategoryId, Model.CategoriesList, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.CategoryId, "", new { @class = "text-danger" })
            </div>
        </div>


        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </div>
    </div>
}

我可以看到ModelState.IsValid返回false，我得到的错误是

The ViewData item that has the key 'CategoryId' is of type 'System.Int32' but must be of type 'IEnumerable<SelectListItem>'.

具有键“CategoryId”的ViewData项的类型为“System.Int32”，但必须为“IEnumerable”类型。

---

请如何解决此问题？

ModelState总是返回false的原因是属性

CategoryName

具有

[Required]

属性，并且您不呈现控件并回发值，因此它总是

null

，因此无效。但是，

CategoryName

与下拉列表关联的选定类别相关，因此您应该删除此属性，只需依赖绑定到下拉列表的

CategoryId

该错误的原因是，当您返回视图时（由于上述问题，该视图总是发生），您没有重新分配

CategoriesList

的值，因此它的

null

和

@Html.DropDownListFor（model=>model.CategoryId，model.CategoriesList，…）

抛出异常

为了避免重复，您可以将公共代码重新分解为私有方法（注意，这假设您将

公共列表分类列表{get；set；}

更改为

公共选择列表分类列表{get；set；}

）

现在可以在GET方法和POST方法中调用它

[HttpGet]
public ActionResult CreatePost()
{
    AddPostsVM model = new AddPostsVM();
    ConfigureEditModel(model);
    return View(model);
}

[HttpPost]
public ActionResult CreatePost(AddPostsVM model)
{
  if (!ModelState.IsValid)
  {
    ConfigureEditModel(model); // repopulate select list
    return View(model); // return the view to correct errors
  }
  // Save and redirect
}

---

在post操作中初始化

模型.CategoriesList

。然后您可以看到验证错误。当

model.CategoriesList

为空时，您会遇到错误，在这种情况下，framework会尝试从

ViewData

读取列表，而该值不是错误消息中指定的预期类型。@Matthew我这样做了，但我没有得到任何错误，除了我自己设置的on之外，如果modelstat无效，我应该得到“无效模型”“除此之外，没有error@ibnhamza，当您返回视图时，

model.CategoriesList

为

null

，因此您会得到该错误。如果返回视图，则必须重新分配

选择列表

@StephenMuecke。您能帮我用正确的代码重写代码吗。我会很感激的。从@ibnhamza开始，在POST方法中，

返回视图（model）行之前，一直在涉猎

，只需复制GET方法中用于分配

model.CategoriesList

的代码，即

var categories=。。。。；model.CategoriesList=
@using (Html.BeginForm()) 
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.LabelFor(model => model.PostTitle, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.PostTitle, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.PostTitle, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.PostContent, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @*@Html.TextAreaFor(model => model.PostContent, new { htmlAttributes = new { @class = "form-control" } })*@
                <textarea class="form-control" name="PostContent" id="PostContent" rows="3"></textarea>
                @Html.ValidationMessageFor(model => model.PostContent, "", new { @class = "text-danger" })
            </div>
        </div>
                <div class="form-group">
            @Html.LabelFor(model => model.CategoryId, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.CategoryId, Model.CategoriesList, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.CategoryId, "", new { @class = "text-danger" })
            </div>
        </div>


        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </div>
    </div>
}

[HttpPost]
public ActionResult CreatePost(AddPostsVM model)
{
    try
    {
        if (ModelState.IsValid)
        {
            var newPost = db.Posts.Create();
            newPost.PostTitle = model.PostTitle;
            newPost.PostContent = model.PostContent;
            FormsIdentity identity = (FormsIdentity)User.Identity;
            int nUserID = Int32.Parse(identity.Ticket.UserData);
            newPost.AuthorId = nUserID;
            newPost.CategoryId = model.CategoryId;
            newPost.DateCreated = DateTime.Now;
            db.Posts.Add(newPost);
            db.SaveChanges();
            return RedirectToAction("Index", "Posts");
        }
        else
        {
            ModelState.AddModelError("", "Invalid Model");

        }

    }
    catch (DbEntityValidationException e)
    {
        foreach (var eve in e.EntityValidationErrors)
        {
            Debug.WriteLine("Entity of type \"{0}\" in state \"{1}\" has the following validation errors:",
                 eve.Entry.Entity.GetType().Name, eve.Entry.State);
            foreach (var ve in eve.ValidationErrors)
            {
                Debug.WriteLine("- Property: \"{0}\", Error: \"{1}\"",
                    ve.PropertyName, ve.ErrorMessage);
            }
        }
        throw;


    }
    return View(model);
}

The ViewData item that has the key 'CategoryId' is of type 'System.Int32' but must be of type 'IEnumerable<SelectListItem>'.

private void ConfigureEditModel(AddPostsVM model)
{
  model.CategoriesList = new SelectList(db.Categories, "CategoryID", "CategoryName");
  // any other common stuff
}

[HttpGet]
public ActionResult CreatePost()
{
    AddPostsVM model = new AddPostsVM();
    ConfigureEditModel(model);
    return View(model);
}

[HttpPost]
public ActionResult CreatePost(AddPostsVM model)
{
  if (!ModelState.IsValid)
  {
    ConfigureEditModel(model); // repopulate select list
    return View(model); // return the view to correct errors
  }
  // Save and redirect
}