为什么我的模型后失去DropDownList的项目？ [英] Why my model in post lose dropdownlist items?

问题描述

我有一个动作一个简单的页面MVC。
在索引操作我创建属性model.categoria的实例获取方法，我和3项值。
现在的问题是，如果我跑行动的岗位指标，因为你可以看到下面，我有一个错误，因为DropDownList的相关model.categoria为空。
我的问题是：
我总是在实例化每一个岗位的model.categoria为了避免这个问题，一方面，今天我不知道，是理所当然的？

下面是code，我将返回一个错误：
值不能为空。

控制器动作：

 公众的ActionResult指数（）
{
    VAR模型=新Models.UploadVideoPage.UploadVideoPageModel（）;
    model.categoria =新的List＆LT; Models.UploadVideoPage.ListCategorie＆GT;（）;
    model.categoria.Add（新ListCategorie {TitoloCategoria =Volontariato，idCategoria = 1}）;
    model.categoria.Add（新ListCategorie {TitoloCategoria =INTERNO，idCategoria = 2}）;
    model.categoria.Add（新ListCategorie {TitoloCategoria =Sindacato，idCategoria = 3}）;
    返回查看（模型）;
}
[HttpPost]
公众的ActionResult指数（Models.UploadVideoPage.UploadVideoPageModel模型）
{
    返回查看（模型）;
}
 

index.cshtml

  @ Html.LabelFor（X =＆GT; x.categoria）
            @ Html.DropDownListFor（X =方式＆gt; x.categoria.First（）idCategoria，新的SelectList（Model.categoriaidCategoria，TitoloCategoria））
 

解决方案

您不要在每次做一回发，除非你把隐藏字段的所有值表单中的时间来填充列表。只需要code几行，虽然使这项工作以不同的方式

 公众的ActionResult指数（）
    {
        VAR模型=新Models.UploadVideoPage.UploadVideoPageModel（）;
        PopulateDependencies（模型）;
        返回查看（模型）;
    }
    [HttpPost]
    公众的ActionResult指数（Models.UploadVideoPage.UploadVideoPageModel模型）
    {
        PopulateDependencies（模型）;
        返回查看（模型）;
    }    私人无效PopulateDependencies（Models.UploadVideoPage.UploadVideoPageModel模型）
    {
        model.categoria =新的List＆LT; Models.UploadVideoPage.ListCategorie＆GT;（）;
        model.categoria.Add（新ListCategorie {TitoloCategoria =Volontariato，idCategoria = 1}）;
        model.categoria.Add（新ListCategorie {TitoloCategoria =INTERNO，idCategoria = 2}）;
        model.categoria.Add（新ListCategorie {TitoloCategoria =Sindacato，idCategoria = 3}）;
    }
 

I have one simple page mvc with one action. Get method in the index action I create an instance of the property model.categoria and I value with 3 items. The problem is that if I run the post index of action, as you can see below, I have an error because the dropdownlist associated model.categoria is null. My question is: I always instantiated at every post the model.categoria order to avoid this problem, one that today I wonder, is rightly so?

Here is the code that I will return an error: Value cannot be null.

controller action:

public ActionResult Index()
{
    var model = new Models.UploadVideoPage.UploadVideoPageModel();
    model.categoria = new List<Models.UploadVideoPage.ListCategorie>();
    model.categoria.Add(new ListCategorie { TitoloCategoria = "Volontariato", idCategoria = 1 });
    model.categoria.Add(new ListCategorie { TitoloCategoria = "Interno", idCategoria = 2 });
    model.categoria.Add(new ListCategorie { TitoloCategoria = "Sindacato", idCategoria = 3 });
    return View(model);
}
[HttpPost]
public ActionResult Index(Models.UploadVideoPage.UploadVideoPageModel model) 
{
    return View(model);
}

index.cshtml

 @Html.LabelFor(x => x.categoria)
            @Html.DropDownListFor(x => x.categoria.First().idCategoria, new SelectList(Model.categoria, "idCategoria", "TitoloCategoria"))

解决方案

You do have to populate the list every time you do a post back unless you were to put hidden field for all of the values in your form. It only takes a few lines of code though to make this work a different way

public ActionResult Index()
    {
        var model = new Models.UploadVideoPage.UploadVideoPageModel();
        PopulateDependencies(model);
        return View(model);
    }
    [HttpPost]
    public ActionResult Index(Models.UploadVideoPage.UploadVideoPageModel model)
    {
        PopulateDependencies(model);
        return View(model);
    }

    private void PopulateDependencies(Models.UploadVideoPage.UploadVideoPageModel model)
    {
        model.categoria = new List<Models.UploadVideoPage.ListCategorie>();
        model.categoria.Add(new ListCategorie { TitoloCategoria = "Volontariato", idCategoria = 1 });
        model.categoria.Add(new ListCategorie { TitoloCategoria = "Interno", idCategoria = 2 });
        model.categoria.Add(new ListCategorie { TitoloCategoria = "Sindacato", idCategoria = 3 });
    }

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