从 ASP.NET MVC 中的 DropDownList 获取默认 NULL 值

我正在创建一个Trailer对于现有的Driver（可以从下拉列表中选择）。

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

对于 CREATE 函数来说它工作得很好。

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

对于编辑功能（编辑拖车号码并离开司机NULL） 这是行不通的。

//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);
}    

我有- 请选择 -在下拉列表中作为第一个空值.
我怎么能放一个NULL第一个空值的值（因此预告片将具有NO从下拉列表中选择驱动程序？

这是因为你的driverID财产在Drivers类不是nullable ?这就是为什么当你选择-- Please Select --它给出验证消息The driverID field is required所以你应该设置driverID to nullable like

public int? driverID {get;set;}

现在它的默认值是null当你选择-- Please Select --

Edit

另一种方法是手动添加默认对象，例如

in view

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

并在编辑操作中

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);
}