C# LINQ - Does Select filter nulls when used with a method group?

Question

Today I wrote some code that takes a list of class A instances and creates a list of class B instances using a static factory method:

public abstract class B
{
    public static B CreateB(A a)
    {
        // return a new B instance or null if a B instance can't be created
    }
}

I needed to filter out the nulls so I used LINQ to do both tasks:

var bList = aList.Select(a => B.CreateB(a)).Where(b => b != null).ToList();

Code runs like expected, but I noticed Resharper was suggesting, on the call to CreateB, I should "Convert to method group". I wondered what that means and found some interesting answers on this site. For example: What is a method group in C#? and a comment by Asad to stitty's answer here Filtering Null values in Select (.Select(x => transform(x)) can be .Select(transform))

So I changed my code to:

var bList = aList.Select(B.CreateB).Where(b => b != null).ToList();

This too works as expected, but now I get to my question. Resharper now tells me the Where lambda expression can be removed because b != null "is always true".

I played around and Resharper is correct, but why? Why is there a difference in the returned collection of the Select() when I use a method group? Why does the Select() seem to filter out the nulls out of the collection when I use a method group?

Just out of curiosity - how does the static method CreateB() 'return a new B instance'? Will it create an inherited class instance? I know it has nothing to do with your question, it's just that I have never seen such a design before (and I'm not an expert, either). — shay__, Jul 16 '15 at 12:13
Are you sure? I just reconstructed your example and I don't get anything about removing the `Where` clause. — Yuval Itzchakov, Jul 16 '15 at 12:15
@shay__ B is an abstract class and CreateB() returns an instance of an inherited class (or null). — meir shapiro, Jul 16 '15 at 12:19
@YuvalItzchakov Well, Resharper doesn't say "Remove the Were clause", it says the "Expression is always true" about the b != null check. — meir shapiro, Jul 16 '15 at 12:21
No I mean, I don't get the ReSharper hint, not the semantics :) Is `A` a reference type? I think we're missing something in your type declaration. Perhaps post your actual types? — Yuval Itzchakov, Jul 16 '15 at 12:23
Yes, A is some custom object I have. B is abstract as I mentioned in an earlier comment, maybe that makes a difference? — meir shapiro, Jul 16 '15 at 12:26
@mDC The aList doesn't contain any nulls. I just can't map all instances of A to a B instance so the CreateB() method returns null in this case. — meir shapiro, Jul 16 '15 at 12:27
@mDC What contents of bList do you get if you replace `CreateB` with `return null;`? — Pavel Krymets, Jul 16 '15 at 12:30

Pavel Krymets · Accepted Answer · 2015-07-16T12:32:21.557

2

Both aList.Select(a => B.CreateB(a)).Where(b => b != null).ToList(); and aList.Select(B.CreateB).Where(b => b != null).ToList(); use the same overload of Select method.

I tested it with LinqPad and got the following output:

LinqPad

So Select doesn't filter anything. What version of Resharper and VisualStudio are you using?

Latest ReSharper on VS2013 doesn't tell anything:

Latest ReSharper on VS2013

edited Jul 16 '15 at 12:32

answered Jul 16 '15 at 12:14

Pavel Krymets

6,253
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Thanks for testing. I'm using VisualStudio 2012 with Resharper 9.0 update 1. – meir shapiro Jul 16 '15 at 12:29
Turns out this is a bug in ReSharper. I debugged the Where clause by changing it to: .Where(item => { return item != null; }) and while Resharper says "Expression is always true" it is actually false. – meir shapiro Jul 16 '15 at 12:47
1

Try upgrading to ReSharper 9.1.1 or the 9.2 EAP. If it's still reported as an error, it's a bug, and should be reported to https://youtrack.jetbrains.com – citizenmatt Jul 16 '15 at 13:25

C# LINQ - Does Select filter nulls when used with a method group?

1 Answers1