Dropdown selected value not shown on edit from

I want to edit my product so I have a three dropdown when I click Edit Button product load and Run this function Edit Product data load in result variable but not shown on the front end. I have done from my side everything is correct but I don't know why the value is not shown in my dropdown during the edit.

Any Expert can see the code and kindly tell me where I am wrong and how to correct this issue.

public class ProductViewModel
       {

          public Product listProducts = new Product();
           public List<ProductDetail> listProductDetails = new List<ProductDetail>();
   }

public class Product
{
         public int P_SizeID { get; set; }
        public int P_Color_ID { get; set; }
        public int PType_ID { get; set; }
}

public class ProductDetails
    {
      Public Int64 ProductID { get; set; }
        public string PDescription { get; set; }
        public string Model { get; set; }
        public string Condition { get; set; }
        public string OS { get; set; }
        public string DualSim { get; set; }
        public string TouchScreen { get; set; }
        public string ScreenSize { get; set; }
    }

public ActionResult EditProduct(int id)  
            {  
                List<ProductType> PTlist = _IproductType.PTList();  
                ViewBag.Ptlist = new SelectList(PTlist, "PType_ID", "P_Name");  

                //Product Color List  
                List<P_Color> pColorList = _IProductColor.PColorlist();  
                ViewBag.pColor_List = new SelectList(pColorList, "C_ID", "C_Name_OR_Code");  

                List<P_Size> pSizeList = _ISize.pSizeList();  
                ViewBag.pSizeLists = new SelectList(pSizeList, "S_ID", "S_Size");  

                var result = _IProducts.GetProductByID(id);  

                return View(result);  
            }  



public ProductViewModel GetProductByID(int ID)
        {
            ProductViewModel listprod = new ProductViewModel();

            try
            {
                var productsl = (from p in _db.Products
                                 where p.ProductID == ID
                                 select p).FirstOrDefault();


                var productD = (from pd in _db.ProductDetails
                                join b in _db.Products on pd.ProductID equals b.ProductID
                                where pd.ProductID == ID
                                select pd).ToList();
                return new ProductViewModel { listProducts = productsl, listProductDetails = productD };
            }

            catch (Exception)
            {

                throw;
            }

<div class="form-group">  
                                    <label class="control-label col-lg-3"><span data-toggle="tooltip" class="label-tooltip" data-original-title="Product type"> Product Category </span></label>  
                                    <div class="col-lg-9">  
                                        <div class="col-sm-9 controls">  
                                            @if (ViewBag.Ptlist != null)  
                                            {  
                                                @Html.DropDownListFor(m => m.listProducts.PType_ID, ViewBag.Ptlist as SelectList, "Choose Product Category", new { @class = "", @id = "PTypeID" })  
                                            }  
                                        </div>  

                                    </div>  
                                </div>  
                                <div class="form-group">  
                                    <label class="control-label col-lg-3"><span data-toggle="tooltip" class="label-tooltip" data-original-title="Product type"> Product Color </span></label>  
                                    <div class="col-lg-9">  
                                        <div class="col-sm-9 controls">  
                                            @if (ViewBag.pColor_List != null)  
                                            {  
                                                @Html.DropDownListFor(m => m.listProducts.P_Color_ID, ViewBag.pColor_List as SelectList, "Choose Product Color", new { @class = "", @id = "PColorID" })  
                                            }  
                                        </div>  

                                    </div>  
                                </div>  
                                <div class="form-group">  
                                    <label class="control-label col-lg-3"><span data-toggle="tooltip" class="label-tooltip" data-original-title="Product type"> Product Size </span></label>  
                                    <div class="col-lg-9">  
                                        <div class="col-sm-9 controls">  
                                            @if (ViewBag.pSizeLists != null)  
                                            {  
                                                @Html.DropDownListFor(m => m.listProducts.P_SizeID, ViewBag.pSizeLists as SelectList, "Choose Product Size", new { @class = "", @id = "PSizeID" })  
                                            }  
                                        </div>  

                                    </div>