DropDownlist in Modal popup - asp.net-core

Question

I am tryingto populate a DropDownlist with the viewbag defined in the controller and i used a modal popup :
ProductController.cs
public IActionResult Create()
{
List<Category> categories = _dbcontext.Category.ToList();
ViewBag.bpCategories = new SelectList(categories, "CategoryId", "CategoryName");
Product product = new Product();
return PartialView("_AddProductPartialView",product);
}
_AddProductPartialView.cshtml:
#model WebApplication1.Models.Product
<div class="modal fade" role="dialog" tabindex="-1" id="addProduct" aria-labelledby="addProductLabel" aria-hidden="true">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title" id="addProductLabel">Products</h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="modal-body">
<form asp-action="Create" method="post">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
......
<div class="form-group">
<label asp-for="CategoryId" class="control-label">CategoryId</label>
#Html.DropDownListFor(m => m.CategoryId, ViewBag.bpCategories as IEnumerable<SelectListItem>, "--Select ---", new { #class = "form-control" })
</div>
...
<div class="modal-footer">
<button type="button" class="btn btn-secondary" data-dismiss="modal">Cancel</button>
<button type="submit" class="btn btn-primary">Save</button>
</div>
</form>
</div>
</div>
</div>
</div>
But i got this error:
The ViewData item that has the key 'CategoryId' is of type
'System.Int32' but must be of type 'IEnumerable'.
So what is the correct way to populate a DropDownlist in a modal popup??
EDIT
I tried also this code but i got an empty Dropdownlist:
<select asp-for="CategoryId" class="form-control" asp-items="ViewBag.bpCategories"></select>

Answer 1

Following code snippet with testing data can populate dropdown well, you can refer to it.
List<Category> categories = new List<Category>
{
new Category
{
CategoryId = 1,
CategoryName = "Category1"
},
new Category
{
CategoryId = 2,
CategoryName = "Category2"
},
new Category
{
CategoryId = 3,
CategoryName = "Category3"
}
};
ViewBag.bpCategories = categories.AsQueryable().Select(s => new SelectListItem { Value = s.CategoryId.ToString(), Text = s.CategoryName }).ToList();
In view page
<select asp-for="CategoryId" class="form-control" asp-items="ViewBag.bpCategories as List<SelectListItem>"></select>
Test Result

Answer 2

In Controller :
List<CollectionDetails> CollectionData = new List<CollectionDetails>();
CollectionData = GetCollectionDetails();
TempData["Department"] = new SelectList(CollectionData, "CollectionId",
"CollectionName");
In ViewModal :
#Html.DropDownListFor(model => model.CollectionId, new SelectList(TempData["Department"] as System.Collections.IEnumerable, "value", "text"), ---please select---, new { #tabindex = 13, #class = "form-control" })