Why is this c# snippet legal?

Question

Silly question, but why does the following line compile?

int[] i = new int[] {1,};

As you can see, I haven't entered in the second element and left a comma there. Still compiles even though you would expect it not to.

http://stackoverflow.com/questions/2311864/history-of-trailing-comma-in-programming-language-grammars — Josh Lee, Mar 02 '10 at 06:31
I'm amazed anyone would be satisfied with the answer "Because that's what the spec says". Steck's and Jimmy's answers explain *why it is beneficial*. — Daniel Earwicker, Mar 02 '10 at 07:16
Oh yes, the arrow is fantastic. No one is complaining about the arrow. — Daniel Earwicker, Mar 02 '10 at 07:27
@Earwicker: The question is not "How can I use this trailing comma?", but "Why does it compile"? From question to answer... — Luca, Mar 08 '10 at 17:59
@Luca - by that reasoning we could have another 1000 questions about each feature of the language, asking "Why does this compile?", each with an answer from you saying "Because the standard says this..." Such an answer says almost nothing. The question contains a big clue: "Still compiles even though you would expect it not to." Actually you should expect it to compile, and a language that didn't compile it would be poorer. It's a feature, not a bug. The good answers explain this. — Daniel Earwicker, Mar 08 '10 at 19:20

Luca · Accepted Answer · 2010-03-08T21:03:36.457

I suppose because the ECMA 334 standard say:

array-initializer:
    { variable-initializer-list(opt) }
    { variable-initializer-list , }
variable-initializer-list:
    variable-initializer
    variable-initializer-list , variable-initializer
variable-initializer:
    expression
    array-initializer

As you can see, the trailing comma is allowed:

{ variable-initializer-list , }
                            ↑

P.S. for a good answer (even if this fact was already pointed by many users). :)

Trailing comma could be used to ease the implementation of automatic code generators (generators can avoid to test for last element in initializer, since it should be written without the trailing comma) and conditional array initialization with preprocessor directives.

score 10 · Answer 2 · answered Mar 02 '10 at 06:35

10

This is syntax sugar. In particular, such record can be useful in code generation.

int[] i = new int[] {
    1,
    2,
    3,
};

Also, when you are writing like this, to add new line you need to add text only in single line.

answered Mar 02 '10 at 06:35

Steck

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Annoying that this is allowed in arrays, but not in Enums, where they would be much more useful. – Jon Limjap Mar 02 '10 at 06:38
IMO, this and Jimmy's answer should be edited together and then the result accepted! And I would add the observation that this is how `;` works between statements, for exactly the same reason. And that in Erlang, where the trailing symbol is different (a dot instead of a comma), it may appear comfortingly similar to English at first but users end up hating it for the same reasons given here. – Daniel Earwicker Mar 02 '10 at 07:24

score 7 · Answer 3 · answered Mar 02 '10 at 06:26

7

It should compile by definition.

There is no second element. A trailing comma is valid syntax when defining a collection of items.

i is an array of int containing a single element, i[0] containing the value 1.

answered Mar 02 '10 at 06:26

Mitch Wheat

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That extra comma tells me another value is expected. – Razor Mar 02 '10 at 06:29
7

That may be your interpretation, but that's not how it is defined. – Mitch Wheat Mar 02 '10 at 06:30
That may be the case but it still doesn't make any sense regardless of the spec. – Razor Mar 02 '10 at 06:32
2

But it is still the answer. That snippet is legal because of the spec, and Jimmy gives the reasoning behind this decision. – Dykam Mar 02 '10 at 06:36
@Sir Psycho: Can't help there I'm afraid, that's how the designers of the language defined it. – Mitch Wheat Mar 02 '10 at 06:36
1

Go yell at everyone who wrote C, Java, and C# to change the spec. – Jon Limjap Mar 02 '10 at 06:37
@Luca: the question was "why does this line compile?": I m pretty sure I explained that. – Mitch Wheat Mar 02 '10 at 06:39
@Sir Psycho: I can't help notice you have accepted the answer that quotes the spec you find so hard to swallow! lol – Mitch Wheat Mar 02 '10 at 06:44
Sir Psycho: When you write `int i; i++;` you don't say that the extra semicolon tells you that another statement is expected, do you? So why not allow the extra comma also? – Gabe Mar 02 '10 at 06:48

score 7 · Answer 4 · answered Mar 02 '10 at 07:26

Another benefit of allowing a trailing comma is in combination with preprocessor directives:

int[] i = new[] {
#if INCLUDE1
   1,
#endif

#if INCLUDE2
   2,
#endif

#if INCLUDE3
   3,
#endif
};

Without allowing a trailing comma, that would be much more difficult to write.

score 5 · Answer 5 · answered Mar 02 '10 at 06:32

5

its so you can do this and copy/paste lines around without worrying about deleting/adding the commas in the correct places.

int[] i = new[] { 
   someValue(),
   someOtherValue(),
   someOtherOtherValue(),

   // copy-pasted zomg! the commas don't hurt!
   someValue(),
   someOtherValue(),
   someOtherOtherValue(),

};

answered Mar 02 '10 at 06:32

Jimmy

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According to a similar post that jleedev pointed out, its to make automatic code generation easier. Just caught me by supprise thats all :-) – Razor Mar 02 '10 at 06:35

score 3 · Answer 6 · answered Mar 02 '10 at 07:29

3

Same goes for enums:

enum Foo
{
  Bar,
  Baz,
};

answered Mar 02 '10 at 07:29

leppie

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Why is this c# snippet legal?

6 Answers6

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