Error if XElement goes to Null

Question

I have the following code:

var te = doc.Descendants("TESTID").Where(x=> (string)x.Attribute("TESTIDNumber")= finalstr).FirstOrDefault();

In the case te is null, then the following gives an exeption:

var ignorete = doc.Descendants("IgnoreTESTID").Where(x=> (string)x.Attribute("TESTIDNumber")== te.FirstAttribute.Value.toString();

The exception is : Object not set to an instance of the Object

How do I make ignorete to null if te goes null ?

what do you expect `null.FirstAttribute.Value.toString()` to do? — Liam, Apr 27 '18 at 08:26
You appear to have posted an observation, not a question. Yes, you're correct, when `te is null` that line of code will generate an exception. And? — Damien_The_Unbeliever, Apr 27 '18 at 08:27
I want to put the `ignorete` to null as well if te goes null — user726720, Apr 27 '18 at 08:27
That ^ is a statement of intent, again not a question...the clue is the lack of a `?` — Liam, Apr 27 '18 at 08:28
I just didn't know how to make it null if `te` goes null, so asking all to help me with this — user726720, Apr 27 '18 at 08:31
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-conditional-operators — TheGeneral, Apr 27 '18 at 08:33
`var ignorete = (te == null) ? null : doc.Descendants("IgnoreTESTID").Where(x=> etc` — John Wu, Apr 27 '18 at 08:33
@JohnWu for C#6 and above `var ignorete = te ?? doc.Descendants("IgnoreTESTID").Where(x=> etc` — SᴇM, Apr 27 '18 at 08:34
@JohnWu & @@SeM: Thanks, that's what I was looking. I tried googling it but didn't knew what was the operator was called. May I know whats the technical term of this operator — user726720, Apr 27 '18 at 08:36
Conditional operator or ternary operator. Added to my answer below. — John Wu, Apr 27 '18 at 08:40
use `te?.FirstAttribute`, that shall do the job. It will then not get an exception and will compare `(string)x.Attribute("TESTIDNumber")` to null, which shall provide an empty collection, then you can use `DefaultIfEmpty` for the null result for the `ignorete` — Mrinal Kamboj, Apr 27 '18 at 08:49

John Wu · Accepted Answer · 2018-04-27T17:10:03.277

2

You just need a guard clause:

public string GetIgnorete()
{
    var te = doc.Descendants("TESTID").Where(x=>  (string)x.Attribute("TESTIDNumber") == finalstr).FirstOrDefault();
    if ( te == null ) return null;
    return doc.Descendants("IgnoreTESTID").Where(x=> (string)x.Attribute("TESTIDNumber") == te.FirstAttribute.Value.toString();
}

Or you could just use the conditional operator:

var ignorete = (te == null) ? null : doc.Descendants("IgnoreTESTID").Where( x => etc –

Note: I took the liberty of changing your = to a ==. The latter is the equality operator which results in a Boolean based on the comparison. The former is the assignment operator whose result is the value of the assignment.

edited Apr 27 '18 at 17:10

answered Apr 27 '18 at 08:37

John Wu

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`(string)x.Attribute("TESTIDNumber")= te.FirstAttribute.Value.toString();`? – Liam Apr 27 '18 at 08:48
The code in this answer will not work. It's using an asignment operator `=` instead of a comparison operator `==` and it's missing a brace `.Where(x=> (string)x.Attribute("TESTIDNumber")= te.FirstAttribute.Value.toString();` – Liam Apr 27 '18 at 10:10

ataraxia · Answer 2 · 2018-04-27T09:06:01.250

0

You can't, because a var must be instatiated so that C# can assign a type. You cannot create a var object and set it to null.

In this case, you would need to declare it as whatever object type doc.Descendants returns. I've declared as a generic object below:

object ignorete = te != null ? doc.Descendants("IgnoreTESTID").Where(x=> (string)x.Attribute("TESTIDNumber")= te.FirstAttribute.Value.toString()) : null;

edited Apr 27 '18 at 09:06

answered Apr 27 '18 at 08:36

ataraxia

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`(string)x.Attribute("TESTIDNumber")= te.FirstAttribute.Value.toString();`? – Liam Apr 27 '18 at 08:48

Error if XElement goes to Null

2 Answers2