ViewBag data from controller is not visible in Partial View

I am not sure, why this is not working it is the correct code and you can pass ViewBag to PartialView with any type of value, check if you are getting data in oRootObject on server at all or not?

If that is still not working try to make dropdown list in C# Controller and pass string on the front end using ViewBag and then render it using @Html.Raw

So, your C# code would be like this

public ActionResult GetAddressOtherDetails(int id, int AddId)
        {
              AddressProviderDetail apd = new AddressProviderDetail();
            using (var context = new bwavenueEntities())
            {
              // some code removed for brevity

                    var Item = new List<ClassForConnectWise.DropdownListForCompany>();
 var str = "<select id='DropdownListSelect' name='DropdownListSelect' style='width:200px'>";
                    JavaScriptSerializer oJS = new JavaScriptSerializer();
                    var oRootObject = oJS.Deserialize<List<ClassForConnectWise.Class1>>(response.Content);
                    if (oRootObject != null)
                    {
                        foreach (var data in oRootObject)
                        {
                            Item.Add(new ClassForConnectWise.DropdownListForCompany { ID = data.id.ToString(), Value = data.name });
 str = str + "<option value='" + data.id.ToString() + "'>" + data.name + "</option>";
                        }
                    }
              str = str + "</select>";
                  //Select List item in ViewBag
                    ViewBag.DropdownListSelect = str;                              
           }
           //apd is Model
            return PartialView(apd);
       }

In View Render it as

@Html.Raw(ViewBag.DropdownListSelect);

This will work for sure, as now ViewBag is string and you can pass it to View in PartialView.