从ASP.NET MVC中的DropDownList获取默认NULL值 [英] Get a default NULL value from DropDownList in ASP.NET MVC

问题描述

我正在为现有的驱动程序(可以从下拉列表中选择)创建一个预告片.

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" })

对于CREATE函数，它可以完美运行.

For CREATE function it works perfectly.

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

对于EDIT功能(编辑预告片编号并保留驱动程序 NULL )，该功能不起作用.

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);
}    

我有-请在下拉列表中选择-作为第一个空值.
我如何在下拉列表中的第一个空值上放置一个 NULL 值(这样预​​告片将选择 NO 驱动程序)?

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?

推荐答案

这是因为您在 Drivers 类中的 driverID 属性不是 nullable吗?因此，当您选择-请选择-时，它会显示验证消息 driverID字段为必填，因此您应将 driverID 设置为可空，例如

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;}

现在，当您选择时，其默认值为 null -请选择-

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

修改

另一种方法是手动添加默认对象，例如

another way is to add a default object manually like

可见

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

并处于编辑状态

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);
}

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