How can I solve a NullReferenceException?

Question

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What is a NullReferenceException in .NET?

I am a beginner in MVC and I have tried to add a dropdown list to save its selected value in database using sql queries but my code throws a NullReferenceException.

Can anyone help me please?

This is the model

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;
    }
}

This is controller

   [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);

    }

This is the view

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

at this line: 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')"; — Tala Ahmad Alzahabi, Jan 31 '13 at 14:01
Are you sure that the `GearType` is not null? What about `GearType.SelectedValue`? — Jim Mischel, Jan 31 '13 at 14:06
It looks like GearType.SelectedValue is null. Could it be that the user hasn't selected anything? — Matthew Watson, Jan 31 '13 at 14:07
yes GearType doesn't select any thing but i dont know why plese help me? — Tala Ahmad Alzahabi, Jan 31 '13 at 14:10

score 0 · Answer 1 · answered Jan 31 '13 at 14:07

0

Most likely the GearType or GearType.SelectedValue are null in this statement.

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')";

answered Jan 31 '13 at 14:07

Marius Bancila

16,053
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ok yes but i dont know why GearType.SelectedValue is null please help me ? – Tala Ahmad Alzahabi Jan 31 '13 at 14:13
do you have a selected item in the list control? – Marius Bancila Jan 31 '13 at 14:56
yes i have selected item in the dropdown list – Tala Ahmad Alzahabi Jan 31 '13 at 15:22
i think SelectedValue is postedback from model to view but i dont lnow how to solve it any help please? – Tala Ahmad Alzahabi Jan 31 '13 at 15:57

score 0 · Answer 2 · edited Jan 31 '13 at 14:34

at this line:

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')";

If GearType is not initialized or null
If GearType.SelectedValue is null

GearType.SelectedValue.ToString() can throw an exception like reference not set to an instance of an object.

To make problem more clear you can control GearType and GearType.SelectedValue before this line.

if(GearType != null && GearType.SelectedValue != null) {
 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')";
}

Thank you for editing. But I don't know how to put code into boxes. @Matteo — aaltintop, Feb 06 '13 at 14:32

How can I solve a NullReferenceException?

2 Answers2