Populate Dropdownlist from Model in ASP.NET MVC - asp.net-mvc

Question

I am working on a CRUD ASP.NET Core MVC application. I have two entities Product and Categrory, i want to populate a DropDownlist from model "Category" in the "Product" View. Here is my code:
public class Product
{
[Key]
public int ProductId { get; set; }
public string ProductName { get; set; }
public string Description { get; set; }
[ForeignKey("CategoryId")]
public int CategoryId { get; set; }
public virtual Category Category { get; set; }
}
public class Category
{
[Key]
public int CategoryId { get; set; }
[Required]
public string CategoryName { get; set; }
public virtual ICollection<Product> Products { get; set; }
}
ProductController.cs:
[HttpGet]
public IActionResult Create()
{
List<Category> categories = _dbcontext.Category.ToList();
ViewBag.bpCategories = new SelectList(categories, "CategoryId", "Category");
Product product = new Product();
return View(product);
}
and in Create.cshtml i used this code to display the Dropdownlist:
<div class="form-group">
<label asp-for="CategoryId" class="control-label"></label>
<select asp-for="CategoryId" class="form-control" asp-items="ViewBag.bpCategories"></select>
</div>
But this code throws Nullreference exception. Any suggestions??

Answer 1

Maybe an error comes from SelectList constructor. Try this:
ViewBag.bpCategories = new SelectList(categories, "CategoryId", "CategoryName");
Use "CategoryName" as text value instead of "Category".There is no Category property in your Category class.
The third parameter is the data text field. Check here:
https://learn.microsoft.com/en-us/dotnet/api/system.web.mvc.selectlist.-ctor?view=aspnet-mvc-5.2#system-web-mvc-selectlist-ctor(system-collections-ienumerable-system-string-system-string)