C# null propagation

Question

Why does this code throw an ArgumentNullException?

string text = null;
var x = text?.Split('#');
var y = x.ElementAtOrDefault(1);   

but this code does not:

string text = null;
var z = text?.Split('#').ElementAtOrDefault(1);

To get the first snippet to work, I have to use null propagation operator (?) on x:

string text = null;
var x = text?.Split('#');
var y = x?.ElementAtOrDefault(1);

but in the second, it's not required:

var z = text?.Split('#').ElementAtOrDefault(1);

Why?

https://dotnetfiddle.net/rCbIMj

Answer 1

It's because of how the expressions are evaluated:

string text = null; // text is null
        
var x = text?.Split('#'); // x is null
        
var y = x.ElementAtOrDefault(1); // null.ElementAtOrDefault throws an NRE

with your other example:

string text = null; // text is null
        
var z = text?.Split('#').ElementAtOrDefault(1); // z is null, nothing past ?. is evaluated because text was null

Fundamentally, the null propagation operator converts this:

Something?.SomethingElse(...)

To this ternary operation:

Something == null ? null : Something.SomethingElse(...)

And so the execution past the ?. operator is short-circuited out (not executed) when the variable is null.