Object reference not set to an instance of an object c# error

Question

I need to check if a record is saved into database or not. If it was saved in database open another form, else show a message that it is not in database.
If the record is not in database I get this error Object reference not set to an instance of an object.
This is my code please help me to find the error here:

string cmdStr = "Select NO from COM_LIST_EXPORT where NO = '" + txtNO.Text + "'";
SqlCommand cmd = new SqlCommand(cmdStr, CN);
int count = (int)cmd.ExecuteScalar();
if (count == Convert.ToInt32(txtNO.Text))
{
    Archive_Sader dd = new Archive_Sader();
    dd.Show();
}

else
{
    MessageBox.Show("please save first");
}

I would have alook at using parameters for your query. That is vulnerable to SQLi — David Pilkington, Nov 15 '13 at 09:04
Where do you create the object `CN`? Also take a look parameterised queries. — jAC, Nov 15 '13 at 09:04
Canonical answer: [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Grant Thomas, Nov 15 '13 at 09:08

score 7 · Accepted Answer · answered Nov 15 '13 at 09:05

7

ExecuteScalar returns null when no records found.

So when trying to cast null -> int you get a NullReferenceException

Try this instead.

int count = Convert.ToInt32(cmd.ExecuteScalar());

Convert.ToInt32 will return 0 when parameter is null.

answered Nov 15 '13 at 09:05

Sriram Sakthivel

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Soner Gönül · Answer 2 · 2013-11-15T09:15:38.423

2

From SqlCommand.ExecuteScalar

Return Value

The first column of the first row in the result set, or a null reference if the result set is empty.

That's why when there is no row as a result, you actually try to convert null to Int32.

Looks like you need to change your line like;

int count = Convert.ToInt32(cmd.ExecuteScalar());

Because Convert.ToInt32 returns 0 when parameter is null.

Return Value

A 32-bit signed integer that is equivalent to the number in value, or 0 (zero) if value is null.

And you should always use parameterized queries. This kind of string concatenations are open for SQL Injection attacks.

For example;

string cmdStr = "Select NO from COM_LIST_EXPORT where NO = @NO";
SqlCommand cmd = new SqlCommand(cmdStr, CN);
cmd.Parameters.AddWithValue("@NO", txtNO.Text);
...

edited Nov 15 '13 at 09:15

answered Nov 15 '13 at 09:04

Soner Gönül

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1

@Downvoter care to comment at least so I can see where I might be wrong? – Soner Gönül Nov 15 '13 at 09:09
1

I don't see any reason to downvote this.. My +1.. – Sriram Sakthivel Nov 15 '13 at 09:12
1

@SriramSakthivel Looks like haters gonna hate `:)` – Soner Gönül Nov 15 '13 at 09:17

Object reference not set to an instance of an object c# error

2 Answers2