Ternary operator associativity in C# - can I rely on it?

Question

Ahh, don't you just love a good ternary abuse? :) Consider the following expression:

true ? true : true ? false : false

For those of you who are now utterly perplexed, I can tell you that this evaluates to true. In other words, it's equivalent to this:

true ? true : (true ? false : false)

But is this reliable? Can I be certain that under some circumstances it won't come to this:

(true ? true : true) ? false : false

Some might say - well, just add parenthesis then or don't use it altogether - after all, it's a well known fact that ternary operators are evil!

Sure they are, but there are some circumstances when they actually make sense. For the curious ones - I'm wring code that compares two objects by a series of properties. It would be pretty nice if I cold write it like this:

obj1.Prop1 != obj2.Prop1 ? obj1.Prop1.CompareTo(obj2.Prop1) :
obj1.Prop2 != obj2.Prop2 ? obj1.Prop2.CompareTo(obj2.Prop2) :
obj1.Prop3 != obj2.Prop3 ? obj1.Prop3.CompareTo(obj2.Prop3) :
obj1.Prop4.CompareTo(obj2.Prop4)

Clear and concise. But it does depend on the ternary operator associativity working like in the first case. Parenthesis would just make spaghetti out of it.

So - is this specified anywhere? I couldn't find it.

Answer 1

Yes, you can rely on this (not only in C# but in all (that I know) other languages with a conditional operator) and your use-case is actually a pretty common practice although some people abhor it.

The relevant section in ECMA-334 (the C# standard) is 14.13 §3:

The conditional operator is right-associative, meaning that operations are grouped from right to left. [Example: An expression of the form a ? b : c ? d : e is evaluated as a ? b : (c ? d : e). end example]

Answer 2

Refer to msdn: http://msdn.microsoft.com/en-us/library/ty67wk28%28VS.80%29.aspx

"If condition is true, first expression is evaluated and becomes the result; if false, the second expression is evaluated and becomes the result. Only one of two expressions is ever evaluated."

Answer 3

It's described here

Answer 4

The assertion that parentheses detract from the readability of the code is a false assumption. I find the parenthetical expression much more clear. Personally, I would use the parentheses and/or reformat over several lines to improve readability. Reformatting over several lines and using indenting can even obviate the need for parentheses. And, yes, you can rely on the fact that the order of association is deterministic, right to left. This allows the expression to evaluate left to right in the expected fashion.

obj1.Prop1 != obj2.Prop1
     ? obj1.Prop1.CompareTo(obj2.Prop1)
     : obj1.Prop2 != obj2.Prop2
           ? obj1.Prop2.CompareTo(obj2.Prop2)
           : obj1.Prop3 != obj2.Prop3
                  ? obj1.Prop3.CompareTo(obj2.Prop3)
                  : obj1.Prop4.CompareTo(obj2.Prop4);