Question

My application is in Asp.Net MVC3 coded in C#.Net.
I have Views which contains DropDownList.These DropDownlist are populated from Certain master.
Below is my Controller Code.

 [HttpPost]
          public ActionResult TestCreate(MainMaster Testmaster)
          {
            var recExists= from c in db.MainMaster
                        where c.CityName == Testmaster.CityName &&                                                 c.StateID==Testmaster.StateID 
                        select c;

             if (nid.Count()>0)
              {
                  ModelState.AddModelError("", "This City Already Exists");
              }
             if (ModelState.IsValid)
             {
                  db.MainMaster.Add(Testmaster);
                  db.SaveChanges();
                  return RedirectToAction("Index");
             }
             else
             {
                 return View(Testmaster);
             } 
          }

Im getting the error only when im trying to enter Duplicate record.Example: If there is already a City named Mumbai of State Maharashtra,and i still try to enter a record with Mumbai,Maharashtra.Then im not able to show the error message ModelState.AddModelError("", "This City Already Exists");.Rather im getting the System.ArgumentNullException: Value cannot be null. error.

Below is my View code.

@Html.DropDownList("CountryID", new SelectList(ViewBag.CountryIDies as System.Collections.IEnumerable, "CountryID", "CountryName"), new { id="Country" })

This is my DropDownlist and im getting error on the same line.
This code works fine when im using it in those views which doesnt contain DropDownList.
I have debugged and checked the values in Testmaster,its giving correct value of the selected combo.
Also i have checked the ViewBag property that is passing the Value to DropDownList.Its passing all the states to the dropdownlist.

Answer 1

As the compiler was reaching the

ModelState.AddModelError("", "This City Already Exists");

it was disposed.Hence my @Html.DropDownList("CountryID", new SelectList(ViewBag.CountryIDies as System.Collections.IEnumerable, "CountryID", "CountryName"), new { id="Country" }) was not having any value in it.
To overcome this i added

   ViewBag.CountryID = new SelectList(db.CountryMasters, "CountryID", "CountryName");

And the issue was sorted.