Why does this code throw "Object reference not set to an instance of an object" exception

Question

I'm trying to update my database using datagridview but it throws "Object reference not set to an instance of an object" exception below is my code:

foreach (DataGridViewRow dgRow in dataGridView1.Rows)
{
   //string re = dgRow.Cells[0].Value.ToString();
   string re =Convert.ToString(dgRow.Cells[0].Value.ToString());
   string strQuery = "UPDATE unit_master SET unit='"+re.ToString()+"'";
   SqlCommand scmd = new SqlCommand(strQuery,SqlConn);
   scmd.ExecuteNonQuery();
}
SqlConn.Close();

Hello. Please use parametrized queries and tell us where the error is. Also, why are you calling `Convert.ToString()` on a string, and `.ToString()` on the string you got from converting the string to a string? — Ry-, Dec 21 '13 at 03:51
Please see [this post](http://stackoverflow.com/questions/1943465/avoiding-null-reference-exceptions). — Brian, Dec 21 '13 at 03:53
actually with above code data is getting updated but only exception getting throw anyway.and i did remove one of the .ToString conversion but then data is not getting updated. — user3115080, Dec 21 '13 at 03:56
You may want to have a "WHERE" clause else you're setting every value in the table to the same. — SQLMason, Dec 21 '13 at 03:57
`Convert.ToString("Use a Debugger and step through it to find out why".ToString());` — Aniket Inge, Dec 21 '13 at 03:58
Use a parametrized SQL instead. Your code is vulnerable to SQL injection. (see here the post from Jeff A. http://www.codinghorror.com/blog/2005/04/give-me-parameterized-sql-or-give-me-death.html) — turtlepick, Dec 21 '13 at 04:04
thanks to all of u some part of it working some of it undone but i will figure it out,if i need some more help i will add a comment,thanks for your help it really helped me — user3115080, Dec 21 '13 at 04:16
Dan Andrews i did use WHERE clause but then data is not getting update,which was getting update before i use WHERE clause,i know there should have WHERE clause,but still i cant figure out what is wrong... — user3115080, Dec 21 '13 at 04:27
http://stackoverflow.com/users/276673/flaviotsf how should i use parametrized SQL since i dont know that much about SQL deeply — user3115080, Dec 21 '13 at 04:29

CodeRedick · Answer 1 · 2013-12-21T03:58:26.287

2

You need to run your code in debug mode (F5 in Visual Studio, or click Debug->Start Debug) and trace it. The debugger should actually stop and show you what is null so you can then figure out why it is null. You can also set a breakpoint at the beginning of the loop and step through each line of the code so you can see what is happening. You can set watches on each variable so you can see the values at every step.

Most likely dgRow is null, or it's not converting to string correctly, or the Value of the column is null.

As a side note you're converting dgRow.Cells[0].Value to string twice... Value.ToString is already doing the conversion, so there is no need to call Convert.ToString() again.

edited Dec 21 '13 at 03:58

answered Dec 21 '13 at 03:52

CodeRedick

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actually i did run code using debugger and showing Object reference not set to an instance of an object exception.yes i m doing twice time conversion because by only that way my data is getting updated – user3115080 Dec 21 '13 at 04:01
So you ran in debug, which is good. When it stopped at the exception did you check the value of each variable to see what was null? – CodeRedick Dec 21 '13 at 04:01

score 1 · Answer 2 · edited Dec 21 '13 at 18:34

1

Problem 1: Please check whether you have initialised your SqlConnection instance variable properly.

Problem 2: The GridView cell may have null . If you call any function on null, it throws a NullReferenceException.

Suggestion: Use parameterised queries to avoid SQL injection attacks.

Try this:

SqlConn = new SqlConnection("/*your connection string*/");
foreach (DataGridViewRow dgRow in dataGridView1.Rows)
{
    if(dgRow.Cells[0].Value!=null)
    {
       string re =dgRow.Cells[0].Value.ToString();
       string strQuery = "UPDATE unit_master SET unit=@unit";
       SqlCommand scmd = new SqlCommand(strQuery,SqlConn);
       scmd.Parameters.AddWithValue("@unit",unit);
       scmd.ExecuteNonQuery();
    }
}
SqlConn.Close();

edited Dec 21 '13 at 18:34

Ry-

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answered Dec 21 '13 at 03:54

Sudhakar Tillapudi

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you could have a where value != null in the foreach, less code. – SQLMason Dec 21 '13 at 04:00
No, you didn't understand: http://stackoverflow.com/questions/9072126/linq-foreach-vs-foreach-if – SQLMason Dec 21 '13 at 04:05
thank u http://stackoverflow.com/users/2057380/sudhakar-tillapudi its working,but still some error occurring but it may some of my mistakes in code.. – user3115080 Dec 21 '13 at 04:14
@user3115080: tell me what is the error you are getting now. if its not object reference expetion please create new quetion all the best. – Sudhakar Tillapudi Dec 21 '13 at 04:16
@user3115080: accept any one of the answer . – Sudhakar Tillapudi Dec 21 '13 at 04:17
http://stackoverflow.com/users/2057380/sudhakar-tillapudi i mnot getting exception after i used above code by dan andrews ,but now my data is not getting updated. – user3115080 Dec 21 '13 at 04:32
i use as it is code above except i add WHERE clause in above query – user3115080 Dec 21 '13 at 04:34
@user3115080: can you please update your question with latest code you are trying? – Sudhakar Tillapudi Dec 21 '13 at 04:37
yes i m opening new question and discussion with my recent code would you be able to see that ? – user3115080 Dec 21 '13 at 04:44
@user3115080: sure , dont forget to accept one of the answers which helped you. – Sudhakar Tillapudi Dec 21 '13 at 04:44
hey there is some problem with ask question part of this site,please wait. – user3115080 Dec 21 '13 at 04:57
hey this site is not allowing me to add more questions. – user3115080 Dec 21 '13 at 04:59
@user3115080: what problem you are facing? – Sudhakar Tillapudi Dec 21 '13 at 05:30

Why does this code throw "Object reference not set to an instance of an object" exception

2 Answers2