Get a default NULL value from DropDownList in ASP.NET MVC

Question

I am creating a Trailer for existing Driver (that can be selected from Drop Down list).

@Html.DropDownListFor(x => x.Driver.driverID, (SelectList)ViewBag.DriverID, "-- Please Select -- ", new { @class = "form-control" })

For CREATE function it works perfectly.

//Create Get
public ActionResult Create()
{
    ViewBag.DriverID = new SelectList(db.Drivers, "driverID", "driverFullName");
    return View();
}

For EDIT function (edit trailers number and leave the driver NULL ) it does not work.

//Edit Get
public ActionResult Edit(int? id)
{
     if (id == null)
     {
         return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
     }
     Trailer trailer = db.Trailers.Find(id);
     if (trailer == null)
     {
         return HttpNotFound();
     }
     ViewBag.DriverID = new SelectList(db.Drivers.ToList(), "driverID", "driverFullName");
     return View(trailer);
}    

I have the -- Please Select -- on the drop down list as a first empty value .
How could I put a NULL value on this first empty value (so the trailer would have NO driver selected) from drop down list?

Answer 1

this is because your driverID property in Drivers class is not nullable ? that is why when you select -- Please Select -- it gives validation message The driverID field is required so you should set driverID to nullable like

public int? driverID {get;set;}

now its default value will be null when you select -- Please Select --

Edit

another way is to add a default object manually like

in view

 @Html.DropDownListFor(x => x.Driver.driverID, (SelectList)ViewBag.DriverID, new { @class = "form-control" })

and in edit action

public ActionResult Edit(int? id)
{
     if (id == null)
     {
         return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
     }
     Trailer trailer = db.Trailers.Find(id);
     if (trailer == null)
     {
         return HttpNotFound();
     }

     var list = db.Drivers.ToList();
     list.Insert(0, new Drivers() {driverFullName = "-- Please Select --"});
     ViewBag.DriverID = new SelectList(list, "driverID", "driverFullName"); //showing the list of drivers on edit page
     return View(trailer);
}

Answer 2

When using the Tag Helpers in ASP.NET Core the following snippet might help:

<select asp-for="DriverId" asp-items="DriverIdList" class="form-control">
  <option value="">-- please select --</option>
</select>

Note the value="" , without this attribute modelbinding will fail to bind to a nullable int.