Deleting a row using entity framework

Question

I'm trying to delete a row from a table in my database but I keep getting a null reference exception.

I know that the code I wrote isn't grabbing the ID number from the ComboBox but I don't know how to fix it.

Here's the code I have:

private void btnDelete_Click(object sender, RoutedEventArgs e)
{
 try
 {
 //select row to delete
 Doctor del = ((Doctor)cbDocIdd.SelectedItem);
 Doctor deleted = (from d in MainWindow.nlh.Doctors
 where d.DoctorID == del.DoctorID
 select d).First();
 //delete row from db
 MainWindow.nlh.Doctors.DeleteObject(deleted);
 //Save to database
 MainWindow.nlh.SaveChanges();
 MessageBox.Show("Doctor deleted");
 this.Close();
 }
 catch (Exception ex)
 {
 MessageBox.Show(ex.Message);
 }
}

Any help would be greatly appreciated.

This is the code used to fill the combobox:

private void Window_Loaded(object sender, RoutedEventArgs e)
 {
 cbDocIdd.DataContext = MainWindow.nlh.Doctors;
 }

• Daniel Petrovaliev: Are you sure the doctor with this id exist.
• Nat: yes. the combobox is populated from the database.
• Gert Arnold: So where does this NRE occur?

Answer 1

You need to use .FirstOrDefault and check whether a valid entity has been found:

//select row to delete
Doctor del = ((Doctor)cbDocIdd.SelectedItem);
Doctor deleted = (from d in MainWindow.nlh.Doctors
 where d.DoctorID == del.DoctorID
 select d).FirstOrDefault();
// check if it even exists!!
if(deleted != null) 
{ 
 //delete row from db
 MainWindow.nlh.Doctors.DeleteObject(deleted);
 //Save to database
 MainWindow.nlh.SaveChanges();
 MessageBox.Show("Doctor deleted");
 this.Close();
}

If you use .First() and the doctor with that given ID doesn't exist, you'll get an exception

Also: make sure the del value is OK and not null before using it in the following statement. Same goes for MainWindow - are you sure this is not null ??

Update: can you try these two lines and tell me what the result is??

//select row to delete
object selectedObj = cbDocIdd.SelectedItem;
if(selectedObj != null)
{
 string typeOfSelectedObj = selectedObj.GetType().Name;
}
Doctor del = ((Doctor)cbDocIdd.SelectedItem);

Is selectedObj something other than null?? And if so: what type is it??

Update #2: Ok - so the doctor does exist and is being returned OK - can you try this for me??

Replace this line:

Doctor del = ((Doctor)cbDocIdd.SelectedItem);

with this instead:

Doctor del = cbDocIdd.SelectedItem as Doctor;

When you run this - is del now something other than null ?

Solution:

In the end, the real reason why this code broke lies in the fact that the call to MainWindow.nlh.Doctors.DeleteObject(deleted); caused an event handler to fire, which included this code:

private void cbDocIdd_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
 // populate with DoctorID from db
 Doctor deletedDoc = ((Doctor)cbDocIdd.SelectedItem);
 tbLastName.Text = deletedDoc.LastName.ToString();
 tbFirstName.Text = deletedDoc.FirstName.ToString();
 tbLicenseNumber.Text = deletedDoc.LicenseNumber.ToString();
}

but in this situation, when a doctor is being deleted, the deletedDoc was returned as NULL, but no check is in place to ensure to access only a non-NULL object ..... therefore, an infamous "null-reference exception" was thrown when trying to access the deletedDoc.LastName property...

• Nat: I've tried it with .FirstOrDefault() and it's not helping because the ID does exist (this is a school project and the combobox is populated straight from the db, with only valid IDs). del comes back null but i can't figure out what I'm doing wrong.
• Nat: This is how it's filled:
• Nat: private void cbDocIdd_SelectionChanged(object sender, SelectionChangedEventArgs e) { //populate with DoctorID from db Doctor deletedDoc = ((Doctor)cbDocIdd.SelectedItem); tbLastName.Text = deletedDoc.LastName.ToString(); tbFirstName.Text = deletedDoc.FirstName.ToString(); tbLicenseNumber.Text = deletedDoc.LicenseNumber.ToString(); }
• Nat: Ok if I'm reading this right, selectedobj shows the right doctor but the type says null. Not sure if that makes sense, as i said, i'm very new at this.
• Gert Arnold: Boy, if only we (or better: the OP) would have known the line that threw the exception! You're a terrier Marc!

Answer 2

"null reference exception" shows when you try to use a method that is containing null value.

Such as if product have null value or assigned to null like product=null then you cannot use method as product.count() or something because null.count() is not supported.

 Doctor deleted = (from d in MainWindow.nlh.Doctors
 where d.DoctorID == del.DoctorID
 select d).FirstOrDefault(); 
use this and check null.