Null reference exception

Question

1.00/5 (4 votes)

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C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Web;
using System.Web.UI.WebControls;
using System.Data.OleDb;
using System.Configuration;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        OleDbConnection con;
        OleDbCommand cmd;
        OleDbDataAdapter da;
        DataSet ds;

        public Form1()
        {
            InitializeComponent();
          
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            con = new OleDbConnection(@"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=Database11.accdb");
            cmd = new OleDbCommand("select * from Table1", con);
            da = new OleDbDataAdapter(cmd);
            ds = new DataSet();
            da.Fill(ds, "Table1");
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string str = "";
            if (checkBox1.Checked == true)
            {
                str = str + "," + checkBox1.Text;
            }
            if (checkBox2.Checked == true)
            {
                str = str + "," + checkBox2.Text;
            }
            if (checkBox3.Checked == true)
            {
                str = str + "," + checkBox3.Text;
            }
            if (checkBox4.Checked == true)
            {
                str = str + "," + checkBox4.Text;
            }
            if (checkBox5.Checked == true)
            {
                str = str + "," + checkBox5.Text;
            }
            if (checkBox6.Checked == true)
            {
                str = str + "," + checkBox6.Text;
            }
            if (checkBox7.Checked == true)
            {
                str = str + "," + checkBox7.Text;
            }
            if (checkBox8.Checked == true)
            {
                str = str + "," + checkBox8.Text;
            }
            label1.Text = str;
         
          
                con.Open();
                cmd.CommandText = "insert into Table1(ID,medNo) values("+int.Parse(textBox1.Text)+",'"+str+"')";
                cmd.ExecuteNonQuery();
                da.Update(ds, "Table1");
                MessageBox.Show("data inserted");
                con.Close();
           
           
        }

      
       
    }
}

Posted 12-Mar-15 23:27pm

MayankSemwal

Updated 13-Mar-15 0:15am

RajeeshMenoth

v3

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Comments

Leo Chapiro 13-Mar-15 5:29am

Try to debug you source code dude, I've heard it helps sometimes ;)

MayankSemwal 13-Mar-15 5:33am

i did that friend,its always get stuck to same error..

Thanks7872 13-Mar-15 5:49am

And why on earth do you think that we can read your mind? Who knows where the error is? Put breakpoint on the line you got the error at. Look for the object which is null. Try to make your code such that it should not be null at that point.

cmd.CommandText.......

What is cmd here?

MayankSemwal 13-Mar-15 5:55am

OleDbCommand cmd;

Thanks7872 13-Mar-15 5:57am

Where is it declared? Did you try steps i suggested you in previous comment? If yes, what did you observe?

MayankSemwal 13-Mar-15 6:01am

yes,i put breakpoint before insert command and a label to print output it is working , all working till that point. but as i remove the breakpoint it shows nullreferenceexception at the start on con.open(); so i guess problem is of database.. i only have two columns ID(autonumber),medNo(text)

Santosh K. Tripathi 13-Mar-15 5:37am

+5 for _duDE_

error is at what line? what are values in variables in that line?

MayankSemwal 13-Mar-15 5:42am

insert into statement

Richard MacCutchan 13-Mar-15 6:11am

Instead of posting these vague replies, please edit your question and show the exact code you are using, and mark the line where the error occurs. A null reference is easy to find with your debugger, and the cause is obvious: you have not initialised a variable or object.

MayankSemwal 13-Mar-15 6:15am

Richard,thats the whole updated code..

Richard MacCutchan 13-Mar-15 6:18am

And where does the error occur?

MayankSemwal 13-Mar-15 6:23am

At con.open() its saying NullReferenceException was unhandled

Richard MacCutchan 13-Mar-15 6:39am

Then it means that the con variable is not set up. Use your debugger to find out why. Check its value in the Form1_Load method and again in the click handler.

Richard MacCutchan 13-Mar-15 6:49am

I have just recreated this (as far as I can) and it works. So there must be something else in your code that we are not seeing.

MayankSemwal 13-Mar-15 6:54am

i debugged it . again nullrefernce

Richard MacCutchan 13-Mar-15 6:58am

OK, but you need to find out why it is null. Is it not getting set up in the first place, or is it getting destroyed by some other piece of code?

MayankSemwal 13-Mar-15 7:02am

Richard,thats why i asked for help not getting the problem.

Richard MacCutchan 13-Mar-15 7:30am

Yes, but you need to understand that the code you have posted does not show the problem. As I said, I have recreated as much as I am able, from what you have posted above, and it works. So you must dig deeper into the actual code that you are running (which must be different to what you have posted) to find out why that variable has not been set up, or has been destroyed.

MayankSemwal 13-Mar-15 7:32am

no,Richard thats the all code of my program nothing other than database except this code.

Richard MacCutchan 13-Mar-15 8:01am

Well I cannot see any way that this code can fail, and my test confirmed that. So there must be something additional that is happening which only you can trace.

CHill60 13-Mar-15 8:13am

Can you confirm that it is still a NullReferenceException that you get and not a OleDbException

MayankSemwal 14-Mar-15 0:53am

yes sure.

2 solutions

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Afzaal Ahmad Zeeshan · Accepted Answer · 2015-03-12T23:29:00

Solution 1

Did you mean to write this?

C#

if (!String.IsNullOrWhiteSpace(str))

To check, if the string is not a null or white space and then execute the command.

Also you should always create parameterized commands, this code is viable and exposed to SQL Injection commands and can cause you data loss. You can read on how to pass parameters, instead of string concatenation on MSDN[^].

Edit

I am sorry for getting too late for getting back to you, because I have to face electric supply failure too much here in my country.

Now, since you've provided the code that you're using. You can see there are 2 possible places where this error might generate, the textBox1.Text, and the str. Since that check I provided didn't work, you need to check for the value of textBox1 if that is also not null.

Again, as many have also said... You should try out debugger to see which of these variables is null at the run-time. You can create breakpoints on the lines (I would add that on the function call to see the function executing in front of my sight) and then press F11 to continue executing statements. This way, at every stage, you will be able to see which of the objects are null, and which objects have a value and thus you can fix it.

Posted 12-Mar-15 23:29pm

Afzaal Ahmad Zeeshan

Updated 13-Mar-15 0:41am

v3

Comments

Santosh K. Tripathi 13-Mar-15 5:34am

+5 from me.

but i'll say, give some time to Mayank to learn by debugging.

MayankSemwal 13-Mar-15 6:31am

help me out because been trying from 55 minutes.

Afzaal Ahmad Zeeshan 13-Mar-15 6:42am

Please see the update to my answer.

Afzaal Ahmad Zeeshan 13-Mar-15 6:42am

Thanks for your vote Santosh.

I have also added that to my answer, to let OP make his a decision to test the application in running mode using debugger.

MayankSemwal 13-Mar-15 6:49am

Afzaal,i am doing it even at debugging mode its again get stuck at con.open().

Afzaal Ahmad Zeeshan 13-Mar-15 6:57am

That means there is some sort of problem with the connection, make sure your server would reply and would establish the connection.

MayankSemwal 13-Mar-15 6:57am

tested its saying the same error at the same position as i told before

Afzaal Ahmad Zeeshan 13-Mar-15 7:03am

Then inside the debugger check why is it being null. Does your attempt to initialize the object gets called and so on. You can see all of this data inside your debugger window.

MayankSemwal 13-Mar-15 7:07am

can you open my code into your visual studio. to find whats going on if possible. because in my lapy i tried many things still the same

Afzaal Ahmad Zeeshan 13-Mar-15 7:08am

No -- Because even if I did, I won't be able to connect to the database that is present in your machine and many other factors would pop up. You need to check for this problem yourself Mayank.

MayankSemwal 13-Mar-15 7:10am

ok,Thanks for your help.

MayankSemwal 13-Mar-15 5:38am

did that too same error

MayankSemwal · Accepted Answer · 2015-06-14T21:05:00

namespace windowfromapplication1
{
public partial class registration : Form
{

C#

private void button3_Click(object sender, EventArgs e)
{
    string str = "";
    if (checkBox1.Checked)
    {
        str += checkBox1.Text+",";
    }
    if (checkBox2.Checked)
    {
        str += checkBox2.Text + ",";
    }
    if (checkBox3.Checked)
    {
        str += checkBox3.Text + ",";
    }
    if (checkBox4.Checked)
    {
        str += checkBox4.Text + ",";
    }
    if (checkBox5.Checked)
    {
        str += checkBox5.Text + ",";
    }
    if (checkBox6.Checked)
    {
        str += checkBox6.Text + ",";
    }
    if (checkBox7.Checked)
    {
        str += checkBox7.Text + ",";
    }
    if (checkBox8.Checked)
    {
        str += checkBox8.Text + ",";
    }
    else
    {
        MessageBox.Show("select medicine");
    }

    DataRow drNewRow = dtMedicineParticular.NewRow();

    drNewRow["medName"] = str;
    drNewRow["medQty"] = txtPackets.Text;

    dtMedicineParticular.Rows.Add(drNewRow);

    gvMedicineParticulars.DataSource = dtMedicineParticular;

    //ddlMedName.Text = "";
    txtPackets.Text = "";
 foreach (DataGridViewRow drvMedicine in gvMedicineParticulars.Rows)
                {
                    MedNo = drvMedicine.Cells[0].Value.ToString();
                    MedPacket = drvMedicine.Cells[1].Value.ToString();

                    string insertQriTreat = "Insert into Treatment_Details(Treat_Med_No,Treat_Issue_Date,Treat_Qty,Treat_Due_Date,Treat_P_ID,Treat_P_Pkg) values ('" + MedNo + "',#" + IssueDateMed + "#," + MedQty + ",#" + dueDate + "#," + PatientID + "," + MedPacket + ")";

                    DBT.exeQuery(insertQriTreat);
                }
}