asp.net mvc model binder does not work for drop down list in edit mode

Question

why model binder does not work on drop down list in edit mode ?

in edit view i write this code and test two different ddl :

@Html.DropDownList("ProductParentCategoryId", null, htmlAttributes: new { @class = "form-control" })
@Html.DropDownListFor(model => model.ProductParentCategoryId, (SelectList)ViewBag.ParentId)

and in my controller

ViewBag.ProductParentCategoryId = new SelectList(_productCategoryService.GetAllProductCategory(), "ProductCategoryId", "ProductCategoryTitle");
ViewBag.ParentId = new SelectList(_productCategoryService.GetAllProductCategory(), "ProductCategoryId", "ProductCategoryTitle");

but all textbox in edit mode fill with model binder but not happening for drop down list.

why ?

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I mean is in edit mode, model binder bind all data from database in textbox and each elements ... but in dropdownlist model binder does not bind data from database as Selected Value into dropdownlist

Answer 1

i find my solution The only thing that should be done in my controller:

[http Get]
    public ActionResult Edit(int id)
    {
        var selectedId = _productCategoryService.GetOneProductCategory(id);

        ViewBag.ProductParentCategoryId = new SelectList(_productCategoryService.GetAllProductCategory(), "ProductCategoryId", "ProductCategoryTitle", (int)selectedId.ProductParentCategoryId);
        ViewBag.GroupFiltersId = new SelectList(_groupFiltersService.GetAllGroupFilter().Where(a => a.GroupFilterParentId == null), "GroupFilterId", "GroupFilterTitle");
        return View(_productCategoryService.GetOneProductCategory(id));
    }

view:

 @Html.DropDownList("ProductParentCategoryId", null, htmlAttributes: new { @class = "form-control" })

Answer 2

I would suggest that you bind to a view model and not use a ViewBag.

But to answer your question, in your first example you didn't pass the items to populate the dropdown (second parameter) you passed a null value instead.

Also for dropdowns I have always used IEnumerable<SelectListItem> as opposed to a SelectList collection.

So in your view model you can create a property like: public IEnumerable<ProductCategory> ProductCategories {get; set;} and bind it to your dropdown as follows:

Html.DropDownListFor(m => m.ProductCategoryId, Model.ProductCategories)

https://msdn.microsoft.com/en-us/library/gg548304(v=vs.111).aspx