? operator error in syntax, error in the else

Question

I have this in my class:

public delegate void OnTrue(String TestPassed);
public event OnTrue OnTrueEvent;

public delegate void OnFalse(String TestPassed);
public event OnFalse OnFalseEvent;

This is my Do method:

public void Do()
{
   bool found=false;
   string p ="";

   // some actions

   found ? (OnTrueEvent != null ? OnTrueEvent(p): ;) :(OnFalseEvent != null ? OnFalseEvent(p): ;);
}

The problem that if in line does not work.

Exploded it should be this:

if (found)
   if (OnFalseEvent != null)
      OnFalseEvent(p);
else
   if (OnTrueEvent != null)
      OnTrueEvent(p);

I would like to know where you are wrong and how correct it should be.

';' it is not a valid terminator after ':' (I do not know what the correct translation is, I have visual studio in Italian) — Gianfrancesco Aurecchia, Apr 10 '18 at 08:18
`? :` is an *operator* which *must return* a value, that's why `: ;` is a *syntax error* — Dmitry Bychenko, Apr 10 '18 at 08:18
Just as a general advice - you are really abusing ternary operators here and sacrificing readability for a few lines of code is a really bad idea. I know it's not your question but I think you shld go with your "exploded" variant. — nbokmans, Apr 10 '18 at 08:19
@Dmitry Bychenko yes but also without ';' however, by saying that ')' is not a valid terminator — Gianfrancesco Aurecchia, Apr 10 '18 at 08:20
Possible duplicate: https://stackoverflow.com/questions/41146905/do-nothing-when-other-side-of-ternary-operator-is-reached — Matt.G, Apr 10 '18 at 08:20
Possible duplicate of [?: ?? Operators Instead Of IF|ELSE](https://stackoverflow.com/questions/6208067/operators-instead-of-ifelse) — SᴇM, Apr 10 '18 at 08:21

Marc Gravell · Answer 1 · 2018-04-10T08:24:08.627

4

exploded it should be this:

and there-in is the error; no, it shouldn't. The conditional operator is always an expression, not a statement. It has no defined meaning in the way you've used it.

If you mean if/else: use if/else.

Note that you can simplify some of the code, though:

(found ? OnFalseEvent : OnTrueEvent)?.Invoke(p);

This works because the conditional part is used as an expression - meaning: the value is consumed. In this case, it is consumed by the null-coalescing invoke.

Note that this would only work if OnFalseEvent and OnTrueEvent have the same delegate type, which they probably should. Action<string> would work fine. If they don't, you'll need the longer:

if(found)
    OnTrueEvent?.Invoke(p);
else
    OnFalseEvent?.Invoke(p);

(your choice of braces and indentation is left as an exercise for the reader)

edited Apr 10 '18 at 08:24

answered Apr 10 '18 at 08:19

Marc Gravell

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Note that `?.Invoke(p)` would only work after C# 6.0. – Nisarg Shah Apr 10 '18 at 08:22
1

@Nisarg As the current version is C#7.2, I guess it's ok to answer on C#6 level. – René Vogt Apr 10 '18 at 08:23
@RenéVogt I'm merely noting the fact so that OP isn't perplexed if that syntax doesn't work with VS 2012 or 13. – Nisarg Shah Apr 10 '18 at 08:26

Dmitry Bychenko · Answer 2 · 2018-04-10T08:31:13.703

As an aleternative for null checking you can assign the fields into do nothing:

public delegate void OnTrue(String TestPassed);
public event OnTrue OnTrueEvent = (item) => { }; // do nothing; not null

public delegate void OnFalse(String TestPassed);
public event OnFalse OnFalseEvent = (item) => { }; // do nothing; not null

...

public void Do() {
  ...

  // No need to check for null here
  if (found) 
    OnFalseEvent(p);
  else
    OnTrueEvent(p); 
}

Since OnTrue and OnFalse are different types you can't put it as

(found ? OnFalseEvent : OnTrueEvent)(p);

since ? : must return the same type for both branches

? operator error in syntax, error in the else

2 Answers2