How to use multi selection in ASP.NET MVC application?
Question
Two days already I am trying to figure out and learn how to build multi-selection dropdown list in ASP.NET Core, that is seeded from db context, and results are used further in the constructor, so I decided to ask this question. I did not find anywhere how to use data from posted form, so maybe you will be able to help me. I belive that I did some mistakes in my POST action in controller. I am not sure how I am supposed to get posted data into the controller.
I was trying various combinations on simple example. Best what I have now is:
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
}
}
}
2 answers
solution1
0 2018-07-16 03:00:24
For selectedItems , I think you need to retrive from ViewBag.Lista instead of productVM.Products . When Posting Form from View to Controller , ProductViewModel.Products will always be null.
I suggest you try workaround below by using ViewBag.Lista .
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);
}
solution2
0 ACCPTED 2018-07-16 06:07:37
The solution is pretty simple & You're almost there. I am going to suggest few of the changes. In ViewModel.cs, it is good idea to populate the list property thru a constructor. It avoids Null Reference exceptions & makes it easy to use the object.
public ProductViewModel()
{
Products=new List<SelectListItem>();
}
public List<SelectListItem> Products { get; set; }
public int[] ProductIDs { get; set; }
In controller.cs, I would make changes to return PopulateProducts to return List Of SelectList, rather than populating view bag. Note: Please close the DB Connection as you will run out connection pools eventually.
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;
}
}
Finally, in index.cshtml, you need to specify the model that is expected for the view & populate product ids from @Model.Products.
@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>
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.