使用 linq 查询不存在的记录说 System.NullReferenceException

Question

Querying my sql table with linq for a record that I know doesn't exist I get the following error System.NullReferenceException: 'Object reference not set to an instance of an object, but.FirstOrDefault() is not supposed to handle it if it doesn't find nothing in database, put the default values?

What I want to do is that if a record does not exist in my database, give a message to the user indicated this, but if the record exists, I want it to bring information

I created an example so as not to attach all my code and express my failure

Code of my main form

namespace Fallas
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void numerofactura_Leave(object sender, EventArgs e)
        {
            var bill = Bill.billfromid (billnumber.Text);
            numerofactura.Text = bill.Numero;
            nombrecliente.Text = bill.Cliente;

        }

Code of my class

namespace Fallas
{

   public class BillModel
    {
        public string Number { get; set; } 
        public string Customer { get; set; }    
        public decimal Subtotal { get; set; }
        public decimal Total { get; set; }  
    }
    public static class Bill
    {
        public static BillModel billfromid(string number) 
    {
        using AppDatabase db = new AppDatabase();
        {
            Bill fct= (from u in db.vta_factura
                                where u.Numero == number
                       select u).FirstOrDefault();
            if (fct.Factura_ID == 0)
            {
                MessageBox.Show("Bill not found!", "Atencion!", MessageBoxButtons.OK, MessageBoxIcon.Information);
                return new Bill();
            }
            return fct;
        }


        }
    }
}

Answer 1

Start from this line:

Bill fct= (from u in db.vta_factura
           where u.Numero == number
           select u).FirstOrDefault();

When the line ends, if no matching record exists the fct variable will be set to null (the default value for any class, as per the "or default" part of " .FirstOrDefault() "). Then on the next line we have this:

if (fct.Factura_ID == 0)

You can't look up the Factura_ID property (or any other property) on a null variable, and so we get the NullReferenceException.

You probably want to do this instead:

if (fct is object)

Or this:

if (fct != null)

is object is a relatively recent pattern for checking null in modern C#. It's supposed to be efficient, helps simplify code for some edge cases around nullable/value types, and expresses it in a positive way (no negation).

Alternatively, you could just return the result from this method, with no check, and let higher-level calling code decide on the best way to respond. Then the method could also be useful in a context where a MessageBox is not appropriate.

Answer 2

.FirstOrDefault(); will return null if no records are found