Get data from SQL table throwing null exception even when try block is present

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In my WinForms app, i have this below code which fetches the lot number from MSSQL table when the button is clicked ,
the function was working fine till yesterday , but from today it is throwing below exception and stopping the application even though try block is present.

System.NullReferenceException: 'Object reference not set to an instance of an object.'

i don't know why is this happening , but i was adding another form called "formedit" to edit the existing record , which is very similar to "FornewEntry" , but with prefilled values of selected datagridview row , maybe i have messed up something while doing the same as this is first application as i am learning C# below is the link to my solution kindly help me .

What I have tried:

private void getlotnumbers()
        {
            try
            {
                SqlConnection con = new SqlConnection(cs);
                SqlCommand cmd;
                con.Open();
                string s = "select LotNumber from Lot_Numbers where CoilNumber = @p1";
                cmd = new SqlCommand(s, con);
                cmd.Parameters.AddWithValue("@p1", coilNoTextBox.Text);
                cmd.CommandType = CommandType.Text;
                int i = cmd.ExecuteNonQuery();
                lotNoTextBox.Text = cmd.ExecuteScalar().ToString();
                con.Close();
            }
            catch(Exception ex)
            {
                MessageBox.Show("Lot Number Not Found:" + ex.Message.ToString(), "Lot Number",
                    MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            
        }

Posted 20-Dec-20 20:18pm

Member 14898617

Updated 20-Dec-20 20:59pm

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OriginalGriff · Answer 1 · 2020-12-20T20:31:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

At a guess, there are no rows in your DB that match the text the user typed - so SQL returns no value. Either test for that instead of automatically calling ToString, or call
ExecuteReader and check the Read method return value.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!