Why does the C# compiler remove a chain of method calls when the last one is conditional?

It comes down to the phrase:

(including evaluation of the receiver and parameters of the call) is omitted.

In the expression:

a.GetB().Hello();

the "evaluation of the receiver" is: a.GetB(). So: that is omitted as per the specification, and is a useful trick allowing [Conditional] to avoid overhead for things that aren't used. When you put it into a local:

var b = a.GetB();
b.Hello();

then the "evaluation of the receiver" is just the local b, but the original var b = a.GetB(); is still evaluated (even if the local b ends up getting removed).

This can have unintended consequences, so: use [Conditional] with great care. But the reasons are so that things like logging and debugging can be trivially added and removed. Note that parameters can also be problematic if treated naively:

LogStatus("added: " + engine.DoImportantStuff());

and:

var count = engine.DoImportantStuff();
LogStatus("added: " + count);

can be very different if LogStatus is marked [Conditional] - with the result that your actual "important stuff" didn't get done.

Should it really behave differently based on whether the receiver of the conditional call is stored in a temporary variable or not?

Yes.

What's the rationale behind the C# compiler not evaluating a.GetB() in this situation?

The answers from Marc and Søren are basically correct. This answer is just to clearly document the timeline.

• The feature was designed in 1999, and the intention of the feature was always to remove the entire statement.
• The design notes from 2003 indicate that the design team realized then that the spec was unclear on this point. Up until this point the specification only called out that arguments would not be evaluated. I note that the spec makes the common mistake of calling the arguments "parameters", though of course one could suppose that they meant "actual parameters" rather than "formal parameters".
• A work item was supposed to be created to fix the ECMA specification on this point; apparently that never happened.
• The first time that the corrected text appears in any C# specification was the C# 4.0 specification, which I believe was 2010. (I do not recall if this was one of my corrections, or if someone else found it.)
• If the 2017 ECMA specification does not contain this correction, then that's a mistake which should be fixed in the next release. Better 15 years late than never, I guess.

I did some digging and found the C# 5.0 language specification did actually already contain your second quote in section 17.4.2 The Conditional attribute on page 424.

Marc Gravell’s answer already shows that this behaviour is intended and what it means in practice. You also asked about the rationale behind this but seem to be dissatisfied by Marc's mention of removing overhead.

Maybe you wonder why it is considered overhead that can be removed?

a.GetB().Hello(); not being called at all in your scenario with Hello() being omitted might seem odd at face value.

I do not know the rationale behind the decision but I found some plausible reasoning my own. Maybe it can help you as well.

Method chaining is only possible if each previous method has a return value. This makes sense when you want to do something with these values, i.e. a.GetFoos().MakeBars().AnnounceBars();

If you have a function that only does something without returning a value you cannot chain something behind it but can put it at the end of the method chain, as is the case with your conditional method since it has to have the return type void.

Also note that the result of the previous method calls gets thrown away, so in your example of a.GetB().Hello(); your the result from GetB() has no reason to live after this statement is executed. Basically, you imply you need the result of GetB() only to use Hello().

If Hello() is omitted why do you need to GetB() then? If you omit Hello() your line boils down to a.GetB(); without any assignment and many tools will give a warning that you are not using the return value because this is seldomly something you want to do.

The reason why you seem to not be okay with this is your method is not only trying to do what is necessary to return a certain value, but you also have a side effect, namely I/O. If you did instead have a pure function there would really be no reason to GetB() if you omit the subsequent call, i.e. if you are not going to do anything with the result.

If you assign the result of GetB() to a variable, this is a statement on it's own and will be executed anyway. So this reasoning explains why in

var b = a.GetB();
b.Hello();

only the call to Hello() is omitted while when using method chaining the whole chain is omitted.

You can also look somewhere entirely different to get a better perspective: the null-conditional operator or elvis operator ? introduced in C# 6.0. Although it only is syntactic sugar for a more complex expression with null checks it allows you to build something like a method chain with the option to short-circuit based on the null check.

E.g. GetFoos()?.MakeBars()?.AnnounceBars(); will only reach it’s end if the previous methods do not return null, otherwise subsequent calls are omitted.

It might be counter-intuitive but try thinking of your scenario as the inverse of this: the compiler omits your calls prior to Hello() in your a.GetB().Hello(); chain since you are not reaching the end of the chain anyway.

Disclaimer

This has all been armchair reasoning so please take this and the analogy with the elvis operator with a grain of salt.