Expression expected in ?: operator

Question

why can't I have something like:

Some_Function ? myList.Add(a) : throw new Exception();

Why can't I throw exception in else part of ?: operator? what is the main purpose of ?: operator?

Suggestion If anybody else wondered about the same thing, beside reading the answers below, i suggest that you read this post as well.. Expression Versus Statement

Because both sides need to return a value of the same type, you can do what you want with if/else. — Ben Robinson, Aug 29 '14 at 11:46
`List.Add` is a void method, so `myList.Add(a)` is not an expression. — Lee, Aug 29 '14 at 11:47
What do u mean? we have ?: operator in C# too.. which part of this is C++ specific? — , Aug 29 '14 at 11:48
@Dena Just because you have the same operator in c++ and c#, it doesn't mean it works the same way in both languages. — Ben Robinson, Aug 29 '14 at 11:49
It's worth mentioning the error message: `Invalid expression term 'throw'` — H H, Aug 29 '14 at 11:55
@Dena, it is not correct to think of the conditional operator shorthand for if/else, as you have learned the 2 are not interchangeable. — Ben Robinson, Aug 29 '14 at 12:19

score 6 · Accepted Answer · answered Aug 29 '14 at 12:01

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The other answers and comments are of course true (we can refer to the documentation), though my understanding of the question is more like, "why does it have to be that way?".

According to the C# 5.0 specification, the conditional operator forms an expression, not a statement. I suspect that the reason why it's not a statement, thus preventing you from doing something like a ? b() : throw e is simply because we already have a statement construct for achieving basically the same thing; namely, if..else.

if (a) { b(); } else { throw e; }

The benefit of the conditional operator is that it can be used within statements or other expressions.

bool? nb = GetValue();
if (nb ?? (a ? b() : c())) { throw e; }

answered Aug 29 '14 at 12:01

Dave Sexton

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+1 for being the only person to make the quite important distinction that the conditional operator requires 2 expressions and not statements. – Ben Robinson Aug 29 '14 at 12:05
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@BenRobinson He doesn't strictly say that, just that it *forms* an expression... (+1 for referencing the spec, though, that's the only real answer to "why doesn't C# do x?") – Rawling Aug 29 '14 at 12:15
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@Rawling, OK but close enough. – Ben Robinson Aug 29 '14 at 12:18
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else, throw e. I can imagine the compiler chucking e through a window. – DidIReallyWriteThat Aug 29 '14 at 12:19
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(╯°□°)╯︵ ǝ (@CalvinSmith suggestions for a unicode broken window welcome) – Rawling Aug 29 '14 at 12:58

score 3 · Answer 2 · edited May 23 '17 at 10:33

Because it is defined as:

null-coalescing-expression ? expression : expression

As you can read in ?: Operator (C# Reference):

The conditional operator (?:) returns one of two values depending on the value of a Boolean expression.

One of both expression is evaluated depending on the boolean value of null-coalescing-expression.

Since neither of your two code fragments are expressions that evaluate to a value (List.Add() is void and an exception doesn't evaluate to a value either), this can't compile.

score 2 · Answer 3 · answered Aug 29 '14 at 11:47

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You need to remember that Ternary operator must return something and List.Add is a void method so it fails. Both the sides must be compatible and should return something.

The MSDN says:

The condition must evaluate to true or false. If condition is true, first_expression is evaluated and becomes the result. If condition is false, second_expression is evaluated and becomes the result.

And myList.Add(a) is not an expression.

answered Aug 29 '14 at 11:47

Rahul Tripathi

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More specifically both things it can return must be compatible types – Ben Robinson Aug 29 '14 at 11:47
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@HenkHolterman no, implicit conversions will be applied, documenation mentions about that – Selman Genç Aug 29 '14 at 11:53

score 1 · Answer 4 · answered Aug 29 '14 at 11:53

See ?: Operator on MSDN. Its clearly explained there.

The conditional operator (?:) returns one of two values depending on the value of a Boolean expression. Following is the syntax for the conditional operator.

condition ? first_expression : second_expression;

The condition must evaluate to true or false. If condition is true, first_expression is evaluated and becomes the result. If condition is false, second_expression is evaluated and becomes the result. Only one of the two expressions is evaluated. Either the type of first_expression and second_expression must be the same, or an implicit conversion must exist from one type to the other. You can express calculations that might otherwise require an if-else construction more concisely by using the conditional operator. For example, the following code uses first an if statement and then a conditional operator to classify an integer as positive or negative.

The conditional operator (?:) returns one of two values depending on the value of a Boolean expression. Following is the syntax for the conditional operator.

The condition must evaluate to true or false. If condition is true, first_expression is evaluated and becomes the result. If condition is false, second_expression is evaluated and becomes the result. Only one of the two expressions is evaluated. Either the type of first_expression and second_expression must be the same, or an implicit conversion must exist from one type to the other. You can express calculations that might otherwise require an if-else construction more concisely by using the conditional operator. For example, the following code uses first an if statement and then a conditional operator to classify an integer as positive or negative.

Expression expected in ?: operator

4 Answers4