c# - 如何解决 NullReferenceException？

Question

可能重复：
.NET 中的 NullReferenceException 是什么？

我是 MVC 的初学者，我尝试添加一个下拉列表以使用 sql 查询将其选定的值保存在数据库中，但我的代码抛出了NullReferenceException.

任何人都可以帮助我吗？

这是模型

public class caradvert
{
    [Required]
    public SelectList GearType { get; set; }

    public int Selected { get; set; }

public caradvert()
{
    List<SelectListItem> listItems = new List<SelectListItem>();
    listItems.Add(new SelectListItem()
    {
        Value = "0",
        Text = "اتوماتيك "
    });


    listItems.Add(new SelectListItem()
    {
        Value = "1",
        Text = "عادي"
    });

        GearType = new SelectList(listItems, "Value", "Text");
}

public int CreatAdvert(int userid)
    {

        SqlConnection objConn = new SqlConnection("Data Source=ADMIN-PC;Initial Catalog=mvc4advertisment;Integrated Security=True");
        SqlCommand objCmd = new SqlCommand();
        objCmd.CommandType = CommandType.Text;
        objCmd.CommandText = "insert into Mercedes(Model,GearType,color,EngineCapacity,CarDescription,Price,Image1Url,Image2Url,Image3Url,Userid,State) values('1','" + GearType.SelectedValue.ToString() + "','r','221','ddd','2','ss','ss','ss','1','False')";
        objCmd.Connection = objConn;
        objConn.Open();
        int count = (int)objCmd.ExecuteNonQuery();
        objConn.Close();
        return count;
    }
}

这是控制器

   [HttpGet]
    public ActionResult CreateAdvert()
    {

        caradvert model = new caradvert();
        List<SelectListItem> listItems = new List<SelectListItem>();
        listItems.Add(new SelectListItem()
        {
            Value = "1",
            Text = "اتوماتيك "
        });


        listItems.Add(new SelectListItem()
        {
            Value = "1",
            Text = "عادي"
        });
        model.GearType = new SelectList(listItems, "Value", "Text");



        return View(model);


    }

    [HttpPost]
    public ActionResult CreateAdvert(caradvert model )
    {
                int _records = model.CreatAdvert(1);
                if (_records > 0)
                {
                    return RedirectToAction("Index", "Account");
                }
                else
                {
                    ModelState.AddModelError("", "لا يمكنك اضافة اعلان");
                }

        return View(model);

    }

这是视图

<%:Html.DropDownListFor(m=>m.Selected,Model.GearType,") %>

Answer 1

GearType此语句中的or很可能GearType.SelectedValue为空。

objCmd.CommandText = "insert into Mercedes(Model,GearType,color,EngineCapacity,CarDescription,Price,Image1Url,Image2Url,Image3Url,Userid,State) values('1','" + GearType.SelectedValue.ToString() + "','r','221','ddd','2','ss','ss','ss','1','False')";

Answer 2

在这一行：

objCmd.CommandText = "insert into Mercedes(Model,GearType,color,EngineCapacity,CarDescription,Price,Image1Url,Imag‌​e2Url,Image3Url,Userid,State) values('1','" + GearType.SelectedValue.ToString() + "','r','221','ddd','2','ss','ss','ss','1','False')"; 

1. 如果 GearType 未初始化或为 null
2. 如果 GearType.SelectedValue 为 null

GearType.SelectedValue.ToString() 可以抛出异常，例如未设置对象实例的引用。

为了使问题更清楚，您可以在此行之前控制 GearType 和 GearType.SelectedValue。

if(GearType != null && GearType.SelectedValue != null) {
 Mercedes(Model,GearType,color,EngineCapacity,CarDescription,Price,Image1Url,Imag‌​e2Url,Image3Url,Userid,State) values('1','" + GearType.SelectedValue.ToString() + "','r','221','ddd','2','ss','ss','ss','1','False')";
}