Question

已经两天了，我试图找出并学习如何在ASP.NET Core中构建多选下拉列表，该列表是从db上下文中获得的，结果在构造函数中得到了进一步的使用，因此我决定问这个问题题。我在任何地方都找不到如何使用已发布表格中的数据的方法，因此也许您可以为我提供帮助。我相信我在控制器的POST操作中犯了一些错误。我不确定如何将发布的数据发送到控制器。

我在一个简单的例子上尝试各种组合。我现在拥有的最好的是：

ViewModel.cs：

public class ProductViewModel
    {
        public List<SelectListItem> Products { get; set; }
        public int[] ProductIDs { get; set; }
    }

Index.cshtml：

<!DOCTYPE html>
<html>
<head>
    <title>Aplikacja</title>
    <meta charset="utf-8">
    <link rel="stylesheet" href="~/dist/allstyles.css" />
</head>
<body>
    <form asp-action="Index" method="post">
        <input type="hidden" asp-for="Products" />

        <div class="form-group">
            <label asp-for="Products">Produkty</label>
            <select asp-for="ProductIDs" class="form-control" asp-items="ViewBag.Lista"></select>
        </div>
        <div class="text-center">
            <button class="btn btn-primary" type="submit">Save</button>
            <a asp-action="Index" class="btn btn-secondary">Cancel</a>
        </div>
    </form>

//ViewBag tests from controller
    @ViewBag.Message  //test1, returns 0
    @ViewBag.Message2 //test2, returns 0

</body>
</html>

Controller.cs

public class ProductController : Controller
{
    private readonly ApplicationDbContext _context; //database context

    public ProductController(ApplicationDbContext context)
    {
        _context = context; //context for controller
    }
    //GET
    public ActionResult Index()
    {
        ProductViewModel productVM = new ProductViewModel(); //viewmodel instance
        PopulateProducts(); //populating dropdown
        return View(productVM); //returning viewmodel
    }
    //POST (I belive that here I have screwed something a lot, I do not understand everything what is going on in this action)
    [HttpPost]
    public ActionResult Index(ProductViewModel productVM)
    {
        PopulateProducts(); 

        if (productVM.ProductIDs !=null)
        {
            List<SelectListItem> selectedItems = productVM.Products.Where(p => productVM.ProductIDs.Contains(int.Parse(p.Value))).ToList();

            ViewBag.Message = selectedItems.Count.ToString(); //returns 0
            ViewBag.Message2 = productVM.Products.Count.ToString();//returns 0
        }
        return View(productVM);
    }

    private void PopulateProducts() //method populating dropdown list
    {

        var techQuery = from t in _context.Products //getting products context from database
                        orderby t.Name //sorting by names
                        select t; //selecting item (??)

        if (techQuery != null) //if db context var is not null...
        {
            ViewBag.Lista = techQuery.Select(n => new SelectListItem { Text = n.Name, Value = n.ProductID.ToString() }); //...creating viewbag that is used for populating dropdown list in view

        }
    }
}

Answer 1

对于selectedItems，我认为您需要从ViewBag.Lista而不是productVM.Products检索。将Form从View发送到Controller时，ProductViewModel.Products始终为空。

我建议您使用ViewBag.Lista在下面尝试解决方法。

    public ActionResult ProductSelect(ProductViewModel productVM)
    {
        PopulateProducts();

        if (productVM.ProductIDs != null)
        {
            List<SelectListItem> selectedItems = ((IEnumerable<SelectListItem>)ViewBag.Lista)
                .Where(p => productVM.ProductIDs.Contains(int.Parse(p.Value))).ToList();

            ViewBag.Message = selectedItems.Count.ToString(); //returns 0
            ViewBag.Message2 = ((IEnumerable<SelectListItem>)ViewBag.Lista).Count().ToString();//returns 0
        }
        return View(productVM);
    }

Answer 2

解决方案非常简单，而且您快到了。我将建议一些更改。在ViewModel.cs中，最好通过构造函数填充list属性。它避免了Null引用异常，并易于使用该对象。

 public ProductViewModel()
    {
        Products=new List<SelectListItem>();
    }
    public List<SelectListItem> Products { get; set; }
    public int[] ProductIDs { get; set; }

在controller.cs中，我将进行更改以返回PopulateProducts以返回SelectList列表，而不是填充视图包。注意：请关闭数据库连接，因为最终将耗尽连接池。

public class ProductController : Controller
{
    private readonly ApplicationDbContext _context; //database context

    public ProductController(ApplicationDbContext context)
    {
        _context = context; //context for controller
    }
    //GET
    public ActionResult Index()
    {
        ProductViewModel productVM = new ProductViewModel(); //viewmodel instance
        productVM.Products=PopulateProducts(); //populating dropdown

        return View(productVM); //returning viewmodel
    }
    //POST (I belive that here I have screwed something a lot, I do not understand everything what is going on in this action)
    [HttpPost]
    public ActionResult Index(ProductViewModel productVM)
    {
        productVM.Products= PopulateProducts();

        if (productVM.ProductIDs != null)
        {
            List<SelectListItem> selectedItems = productVM.Products.Where(p => productVM.ProductIDs.Contains(int.Parse(p.Value))).ToList();

            ViewBag.Message = selectedItems.Count.ToString(); //returns 0
            ViewBag.Message2 = productVM.Products.Count.ToString();//returns 0
        }
        return View(productVM);
    }

    private List<SelectListItem> PopulateProducts() //method populating dropdown list
    {

        var techQuery = from t in _context.Products //getting products context from database
            orderby t.Name //sorting by names
            select t; //selecting item (??)

        if (techQuery != null) //if db context var is not null...
        {
            return techQuery.Select(n => new SelectListItem { Text = n.Name, Value = n.ProductID.ToString() }).ToList(); //...creating viewbag that is used for populating dropdown list in view

        }

        return null;
    }
}

最后，您需要在index.cshtml中指定用于查看的模型，并从@ Model.Products填充产品ID。

@model ProductViewModel
<form asp-action="Index">
<div class="form-group">
    <label asp-for="Products">Products</label>
    <select asp-for="ProductIDs" class="form-control" asp-items="@Model.Products"></select>
</div>
<div class="text-center">
    <button class="btn btn-primary" type="submit">Save</button>
    <a asp-action="Index" class="btn btn-secondary">Cancel</a>
</div>

如何在ASP.NET MVC应用程序中使用多重选择？

2 个答案: