C# 6.0 multiple identical null conditional operator checks vs single traditional check

Question

Which out of the following two equivalent ways would be best for the null conditional operator in terms of primarily performance and then ease of use or clarity etc.?

This:

idString = child?.Id;
fatherName = child?.Father?.Name;
motherName = child?.Mother?.Name;

or (assuming that all local vars are null already) this:

if (child != null)
{
    idString = child.Id;
    fatherName = child.Father?.Name;
    motherName = child.Mother?.Name;    
}

Is performance even an issue?

Answer 1

Is performance even an issue?

Short answer: Performance about null-checks will never be an issue in a normal application. It's only a matter of readability and maintainability.

Performance:

Yes, you have 3 "explicit" checks against 1. But you must keep in mind that:

1. The system performs an "implicit" null-check every time you reference an object instance, as explained here, and afaik the JIT doesn't optimize null-checks at all. So in your case the rate is not just 3 against 1.
2. A null-check is a very (really, very) cheap operation, compared with most operations you do in a normal software flow (instance heap allocation, mathematic calculations, a linq query, graphic object rendering, string parsing, ...).

I see only a remote possibility of significant performance differences: if child or Mother or Father are not local variables or simple plain properties, but methods and properties with very long execution time. For example, a GetChild() method that takes 2 sec to execute. Can you see a realistic scenario for that? I can't. Even if that's the case, you can call GetChild() one single time and assign it to a local variable, then call child? 3 times.

Readability:

A single initial if allows to mentally separate different chunks of code. Pretend to be the reader of the code, without knowing anything else: ask yourself if is it simpler to read "if child is not null do all this operations and stuffs, otherwise just move on", or "if child is not null check the Name. If again child is not null check the Mother. If the Mother is not null get the Mother's Name. Then if again child is not null check the Father. If Father is not null... ... ... ".

Maintainability:

Aka, in this case, the DRY principle. For example, why would you repeat the null-check 3 times? Pretend that at a certain point in the future your boss asks to you a code change: it is required not only to check the nullity of a child, but also if its Id is 0 (things like that happen very frequently in any software development process). In your first code section you should correct 3 lines. In your second code section you should correct only 1 line: the initial if.

EDIT:

For a discussion about thread-safety on the null conditional operator, see this question.

Answer 2

In second code example for variables will not set new values, but in first example they set as null or value from specified properties.

The ?. operator names like null-conditional operator. This operator works like:

With using ?.:

var result = someProperty?.someField;

Without using ?.:

if (someProperty != null)
    result = someProperty.someField;
else
    result = null;

About this operator you can read here: https://msdn.microsoft.com/en-us/library/dn986595.aspx.

It is better to use it in fluent of methods invoking. In your sample better to use second variant, because if child is null, then other actions are not perform.