C# Conditional Operator Not a Statement?

Question

I have a simple little code fragment that is frustrating me:

HashSet<long> groupUIDs = new HashSet<long>(); groupUIDs.Add(uid)? unique++ : dupes++; 

At compile time, it generates the error:

Only assignment, call, increment, decrement, and new object expressions can be used as a statement

HashSet.Add is documented to return a bool, so the ternary (?) operator should work, and this looks like a completely legitimate way to track the number of unique and duplicate items I add to a hash-set.

When I reformat it as a if-then-else, it works fine.

Can anyone explain the error, and if there is a way to do this as a simple ternary operator?

Answer 1

According to the error message the ternary operator cannot be used as a statement. You would need to do something like this to turn it into an assignment:

int dummy = groupUIDs.Add(uid)? unique++ : dupes++; 

That being said, I'd recommend to just use if-then-else. It's less confusing because it doesn't involve the creation of "magic" dummy variables...

Answer 2

As others have pointed out, the conditional operator is not a legal statement expression. (The legal statement expressions are assignments, calls, increments, decrements and constructions.)

However, there's a stylistic problem here as well. In my opinion, expressions should be useful for their values, and statements should be useful for their side effects. What you are running into is that you have an expression that is only useful for its side effect, and that is a bad code smell.

You have a side effect, so use a conditional statement rather than a conditional expression.