Why does the null-conditional operator change regular property access?

Question

I'm confused about how the null-conditional operator cascades with normal property access. Take these two examples:

a?.b.c
(a?.b).c

I would expect them to be equivalent: first, the value of a?.b is evaluated, then result.c is evaluated. Thus if a == null, an exception should be thrown.

However, that only happens in the second expression. The first expression evaluates to null, meaning it's the same as a?.b?.c. Why?

Nope, it stops at ?. if a is null. The second one ends up telling it you definitely want to evaluate c. — juharr
– juharr, Commented Aug 1, 2018 at 18:59
@FrankerZ The official docs call it the null-conditional operator, which is why I used that term. — BlueRaja - Danny Pflughoeft
– BlueRaja - Danny Pflughoeft, Commented Aug 1, 2018 at 18:59
@BlueRaja-DannyPflughoeft The important part to notice in the docs is The null-conditional operators are short-circuiting. If one operation in a chain of conditional member access and index operation returns null, then the rest of the chain’s execution stops — juharr
– juharr, Commented Aug 1, 2018 at 19:01

Camilo Terevinto · Accepted Answer · 2018-08-01 19:08:28Z

9

That's only a matter of operator precedence. Let's go through the cases:

a?.b.c

Evaluate a => null is returned, nothing else is evaluated given that the null-conditional operators are short-circuiting.

(a?.b).c

Evaluate a => null is returned
Evaluate ((B)null).c => NullReferenceException is thrown

For these cases to be equivalent, you should be comparing

a?.b.c
(a?.b)?.c
a?.b?.c (as you already mentioned)

edited Aug 1, 2018 at 19:08

answered Aug 1, 2018 at 19:03

Camilo Terevinto

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Grant Winney · Accepted Answer · 2018-08-01 19:30:29Z

I don't know if this'll help or not, beyond what Camilo already provided, but here's a brief comparison showing how the logic might look without the null-conditional operator in play.

public class Program
{
    public static void Main()
    {
        A a;

        a = new A { b = new B { c = 5 } };

        Console.WriteLine(a?.b.c);        // returns 5;
        Console.WriteLine((a?.b).c);      // returns 5;

        a = null;

        Console.WriteLine(a?.b.c ?? -1);  // returns -1;
        Console.WriteLine((a?.b).c);      // throws NullReferenceException


        // Similar to a?.b.c

        if (a != null)
            Console.WriteLine(a.b.c);     // returns 5;
        else
            Console.WriteLine(-1);        // returns -1;


        // Similar to (a?.b).c

        B tmp;
        if (a != null)
            tmp = a.b;
        else
            tmp = null;

        Console.WriteLine(tmp.c);         // returns 5 or throws NullReferenceException
    }
}


public class A
{
    public B b { get; set; }
}

public class B
{
    public int c { get; set; }
}

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