Question

I am building a Windows forms application using VS2010. I want to read the value the user selected from the dropdown ComboBox and generate patientNo based on that. But when i run the application I get a NullReferenceException.

Here is my code:

private void button1_Click(object sender, EventArgs e)
{
    string patientNumber;

    string gender = comboBox2.SelectedValue.ToString();
    if (gender != null)
    {
        if (gender == "Female")
        {
            var generator = new PatientNumberGenerator();
            patientNumber = generator.GeneratePatientNumber(Gender.Female);
            const string message = "patientNumber";

            const string caption = "Testing PatientNumber class";
            var result = MessageBox.Show(message, caption,
                                         MessageBoxButtons.YesNo,
                                         MessageBoxIcon.Question);

        }
        else if (gender == "Male")
        {
            var generator = new PatientNumberGenerator();
            patientNumber = generator.GeneratePatientNumber(Gender.Male);
            const string message = "patientNumber";

            const string caption = "Testing PatientNumber class";
            var result = MessageBox.Show(message, caption,
                                         MessageBoxButtons.YesNo,
                                         MessageBoxIcon.Question);
        }
   }

}

Answer 1

You haven’t said which line the exception is occuring on, so it’s a bit of a shot in the dark. However:

string gender = comboBox2.SelectedValue.ToString();

comboBox2.SelectedValue might be null, in which case trying to call ToString() on it would cause a NullReferenceException. Try:

if (comboBox2.SelectedValue == null)
{
    return;
}
string gender = comboBox2.SelectedValue.ToString();
//[etc…]

If that’s not your issue, the exception may be inside other calls in that method, such as GeneratePatientNumber.