c# Null value returning from Linq

Question

Currently have a list that you are able to add customer details to through user input. later in the software I then use linq to search the list in order to find if the customer details already exist in the list. I currently have an if statement (listed below) that takes the return value from linq and displays if the customer has details already stored on the list.

    Customer cust = finder.list.where(c => c.ID.ToString() == textBox.txt).FirstOrDefault();

    if(cust.ID == null)
    {
        lstDisplay.items.add("Customer details not stored")
    }

The issue I am having is that the program crashes once it reaches the if statement with the error: 'Object reference not set to an instance of an object.'

train was null.

how to I handle null being returned.

If null is returned to the value train.ID I want to compare it to null like I have in the if statement. If it is null then I want to be able to display the message box Like I have done

Your trying to see if cust.ID is null, you need to check if the entire CUST object is null — Brad, Nov 12 '18 at 17:23
Is linq aware that the ID field in the customer table is nullable? i.e. "int?" (question mark intended) — Phill, Nov 12 '18 at 17:27
Also in your `Where` condition, if `ID == null`, then this will fail before you ever get to the `if` statement: `c.ID.ToString()`. So you should change that part as well: `.Where(c => c.ID?.ToString() == textBox.txt)` — Rufus L, Nov 12 '18 at 17:49

score 1 · Accepted Answer · edited Nov 12 '18 at 18:47

1

You need to check if cust is null

if(cust?.ID == null)
{
    lstDisplay.items.Add("Customer details not stored")
}

edited Nov 12 '18 at 18:47

JuanR

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answered Nov 12 '18 at 17:24

Sean T

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Checking for `null` on `cust` should be enough: `if(cust == null)`. – JuanR Nov 12 '18 at 17:26
@JuanR Should be, you'd think ID was a required field :) – Sean T Nov 12 '18 at 17:32
@JuanR yeah, in the OP's original code, if `ID == null`, then the `c.ID.ToString()` call in the `Where` condition will fail before the `if` statement is ever reached... – Rufus L Nov 12 '18 at 17:48
Also, `Add` should be capitalized... – Rufus L Nov 12 '18 at 17:53

c# Null value returning from Linq

1 Answers1