ModelState.IsValid remains false for DropDownList, even though values to properties are binded and p-CodePudding

I am not sure why the ModelState validity checks keep returning false, hence preventing a write to DB action, even though values supplied to all the properties are of right DataType and successfully passed to the HttpPost method. Here is my code below:

**My Models**

   /*  public class Product
    {
        [Key]
        public int ProductID { get; set; }
        public string ProductName { get; set; }
        public string ProductDescription { get; set; }

        public int CategoryID { get; set; }

        public Category Category { get; set; }
    }
    
     public class Category
    {
        [Key]
        public int CategoryID { get; set; }
        public string CategoryName { get; set; }
    } */
    

**My ViewModel**

/*  public class ProductViewModel_02
    {
        public string ProductName { get; set; }
        public string ProductDescription { get; set; }

        [Required(ErrorMessage = "..")]
        public int? CategoryID { get; set; }

        public IEnumerable<SelectListItem> CategoryList { get; set; }
    }
*/

**My Controller**

/*
 [HttpGet]
        public IActionResult Create()
        {
            var model = new ProductViewModel_02()
            {
                CategoryList = GetCategoriesListItems()
            };
            return View(model);
        }

        [HttpPost]
        public IActionResult Create(ProductViewModel_02 pvm)
        {
            if(ModelState.IsValid)
            {
                return RedirectToAction("Index");
            }

            pvm.CategoryList = GetCategoriesListItems();
            return View(pvm);
        }



        private IEnumerable<SelectListItem> GetCategoriesListItems()
        {
            return db.Categories.Select(x => new SelectListItem
            {
                Value = x.CategoryID.ToString(),
                Text = x.CategoryName
            });

        }
*/

**my View Page**

/*
 @Html.DropDownListFor(x => x.CategoryID, new SelectList(Model.CategoryList, "Value", "Text"),
        "Select", new{})#
*/

ModalState is always invalid when form is submitted, below is the result of my debugging. The List object is null and Invalid, amongst the rest of the properties.[enter image description here](https://i.stack.imgur.com/QcG48.jpg)

CodePudding user response：

why the ModelState validity checks keep returning false

I test your code , find you bind values to properties(eg CategoryID) successfully, but you don't give the value to the CategoryList from view to controller. The CategoryList's value is null, so the ModelState validity checks keep returning false.

Below is a way to solve the problem, hope it can help you, try to remove below line from your YourProjectName.csproj file

 <Nullable>enable</Nullable>

Read this to know more.