Resharper possible null reference warning on XElement Element method

Question

Resharper gives a possible null reference warning for the Element method call just after it was cheked for null (TransformElementBad method), but everything is ok if I keep element value in some variable. Is ReSharper right and I should use its suggestion or it is a bug?

// JetBrains ReSharper 8.2.1 
// Build 8.2.1000.4556 on 2014-05-19T09:12:38

public class Transformer
{
    private void TransformElementBad(XElement field)
    {
        var format = string.Empty;

        if (field.Element("ViewFormatInfo") != null)
            format = field.Element("ViewFormatInfo").Value.Trim(); // ![got][1] warning here

        Console.WriteLine(format);
    }

    private void TransformElementGood(XElement field)
    {
        var format = string.Empty;

        var element = field.Element("ViewFormatInfo");
        if (element != null)
            format = element.Value.Trim();

        Console.WriteLine(format);
    }
}

score 1 · Answer 1 · answered Sep 18 '14 at 12:40

1

The Value can be null that is why it is giving the warning, so you have to check for null before calling Trim(), if it is sure that there will always be Value in it then no need of null check.

format = field.Element("ViewFormatInfo").Value !=null ? field.Element("ViewFormatInfo").Value.Trim() : string.Empty;

answered Sep 18 '14 at 12:40

Ehsan Sajjad

61,834
16
105
160

@Ehsan take a look at screenshot http://i.stack.imgur.com/165MA.png Its doesnt hightlighted "Value", only Element() call – bonzaster Sep 18 '14 at 12:55
1

you are not checking for field to be null ``if(field !=null)`` at first – Ehsan Sajjad Sep 18 '14 at 12:56
@ehsan-sajjad it doesn't help either, still get the warning – bonzaster Sep 18 '14 at 13:05
where have you put that? – Ehsan Sajjad Sep 18 '14 at 13:06
`Value` does not return `null`. So this answer is incorrect. – default Sep 18 '14 at 19:49

score 1 · Accepted Answer · answered Sep 18 '14 at 19:27

1

Answer from vendor: Element method is not pure and could not return the same result given the same arguments. So you have to copy its result into local variable if you want to avoid this warning https://youtrack.jetbrains.com/issue/RSRP-424149

answered Sep 18 '14 at 19:27

bonzaster

313
2
12

score 0 · Answer 3 · answered Aug 16 '19 at 15:09

I ain't mean to do gravedigging here, but as no solution was present (only the explanation of this behavior), might as well share my thoughts with this thread.

So, while I cannot argue with JetBrains' position about this question, I noticed that you can avoid this annoying warning in C# 6 and later with null-conditional access:

var format = field.Element("ViewFormatInfo")?.Value.Trim();

Also, note that

The null-conditional operators are short-circuiting. That is, if one operation in a chain of conditional member or element access operations returns null, the rest of the chain doesn't execute.

That means, calling Trim() method on Value property with null-condition will not throw NullReferenceException (as if Value itself was null), but simply returns null to the caller;

Resharper possible null reference warning on XElement Element method

3 Answers3