NullReferenceException was unhandled - How to resolve it

Question

Here is my code:

private void button1_Click(object sender, EventArgs e)
{
    SqlConnection con = new SqlConnection("Data Source=MANINOTEBOOK\\SQLEXPRESS;Integrated Security=SSPI;Initial Catalog=Casesheet");
    con.Open();
    SqlCommand cmd = new SqlCommand("select PatientID from FTR where PatientID='" + textBox1.Text + "'", con);
    textBox2.Text = cmd.ExecuteScalar().ToString();
    if (textBox2.Text == textBox1.Text)
    {
        Consultation cs = new Consultation(textBox1.Text);
        cs.Show();
    }
    else
    {
        MessageBox.Show("Data not found");
    }            
}

When i compile this code i get an error "NullReferenceException was unhandled". I dont know how to resolve it. I need to check the value generated in the "execute scalar" command is null or not. Kindly help me in resolving this problem.

Do you get an error when you **compile** the code, or when you **run** the code? Since it's probably when you **run** the code, please indicate what line of code is causing the error. Your stack-trace, or your debugger will assist you with this. — RB., Oct 30 '12 at 13:30
I doubt that you get the error when you *compile* the code. I suspect you get the error when you *run* the code. — Jon Skeet, Oct 30 '12 at 13:30

Tim Schmelter · Answer 1 · 2012-10-30T13:36:29.630

Most likely the exception was raised on ToString:

textBox2.Text = cmd.ExecuteScalar().ToString();

That happens when no patient with that ID was found in database because then ExecuteScalar returns null. So you should check for null:

Object patID = cmd.ExecuteScalar();
if(patID != null)
{
    String patientID = patID.ToString();
    // ...
}

Note: You should not concatenate strings to build your sql query but use SqlParameters instead to avoid SQL-Injection.

score 1 · Answer 2 · answered Oct 30 '12 at 13:31

1

cmd.ExecuteScalar()

this can return null if the result was empty. you should change it to

 (cmd.ExecuteScalar() ?? "").ToString()

so it is changed to an empty string if it is null

answered Oct 30 '12 at 13:31

user287107

9,286
1
31
47

score 0 · Answer 3 · edited May 23 '17 at 12:33

0

There is a difference* between .ToString() and Convert.ToString()

Convert.ToString() handles null values, i.e. If you give null input, it would give you back an empty string.

Whereas, .ToString() Which you've used - throws an exception when a null value is passed.

so, you could simply choose to rephrase your code like so:

textBox2.Text = cmd.ExecuteScalar().ToString();

** Not the only difference. Read more here: Difference between Convert.ToString() and .ToString()

edited May 23 '17 at 12:33

Community

1
1

answered Oct 30 '12 at 13:35

tempidope

823
1
12
29

That's *a* difference between the two, but not the only difference. – Servy Oct 30 '12 at 13:35
Agreed. I felt in the scope of this question, this difference is important. Thanks for mentioning, I'll add a disclaimer. – tempidope Oct 30 '12 at 13:36

score 0 · Answer 4 · answered Oct 30 '12 at 13:40

private void button1_Click(object sender, EventArgs e)
{
    SqlConnection con = new SqlConnection("Data Source=MANINOTEBOOK\\SQLEXPRESS;Integrated Security=SSPI;Initial Catalog=Casesheet");
  try
  {
    con.Open();
    SqlCommand cmd = new SqlCommand("select PatientID from FTR where PatientID='" + textBox1.Text + "'", con);
    textBox2.Text = cmd.ExecuteScalar().ToString();
    if (textBox2.Text == textBox1.Text)
    {
        Consultation cs = new Consultation(textBox1.Text);
        cs.Show();
    }
    else
    {
        MessageBox.Show("Data not found");
    }    
  } 
   catch(NullReferenceException ex)
   {
   Console.Write(ex.message);
   }     
}

you can catch your exception by putting try catch block and if you want to find out value generated then put breakpoint in code.

NullReferenceException was unhandled - How to resolve it

4 Answers4