Proposal: Permit trailing commas in method signatures and invocations

[MgSam]

Migrated from dotnet/roslyn#12496

Proposal

Allow trailing commas in method/constructor declarations and invocations.

void Foo(
    int bar, 
    int baz,
) { }

Foo(
    bar: 5,
    baz: 3,
);

Rational

One very small but appreciated feature in C# is the tolerance of the compiler for trailing commas in array initialization or object initialization:

var a = new int[] 
{ 
    3, 
    4, 
    5, 
};
var b = new 
{ 
    Bark = "woof", 
    Bite = "chomp", 
}

This makes it easier to reorder or edit the item order without taking time to make sure that last comma gets deleted.

It would be nice if this feature also extended to method signatures and invocation. It would be especially useful in the post C# 4.0 where named parameters can be reordered or used to make a highly descriptive method call.

Example

void Foo (int bar, String baz, Func<int> cat, Action<String> dot, int elf) { }

Foo(
    bar: 5, 
    baz: "",
    cat: () => 6,
);

Wait, having cat last is confusing, let me place it after bar...

//Rather than just copy and paste the relevant line...
Foo(
    bar: 5, 
    cat: () => 6, //Now I need to add this comma here!
    baz: "" //And I need to remove this comma here!
);

Instead, if method invocations were also tolerant of an extra comma, it would make editing easier:

Foo(
    bar: 5, 
    cat: () => 6, 
    baz: "", //Extra comma here, it's not hurting anyone!
);

Final thoughts

TypeScript has and soon JavaScript will have this feature. I think it will work great in C# as well.

[DavidArno]

    baz: "", //Extra comma here, it's not hurting anyone!

It's hurting readability, simply to make the lives of lazy developers easier. It's bad enough that C# already supports superfluous commas in some cases already. Extending that to other cases would be very sad development.

[Pzixel]

@MgSam it's easy to write and hard to read. As you know, 80% of time you are reading the code. Thus this should be never get implemented.

[jveselka]

@DavidArno

It's hurting readability

How? Whats so confusing about it?

...simply to make the lives of lazy developers easier

Of course - thats what all language features are about

It's bad enough that C# already supports superfluous commas in some cases already.

Its already allowed at least on enums and object initializers and how is it bad there? Did it ever caused you any issues?

Frankly, I do not understand why would anyone oppose this improvement.

[wanton7]

I think IDE should be responsible of adding and removing commas when you paste something. Example if you paste something inside list or array initialization. But this shouldn't be added to language.
TypeScript probably added this because it's in some future JavaScript spec.
This just adds noise to language (hurts readability).
This probably wouldn't be so big of an issue if C# had standard formatter like golang has. If you add this that comma will be all over the place.

I can see this happening

var a = new int[] 
{ 
    3, 
    4, 
    5
, 
};

[alrz]

Trailing commas are usually allowed when it doesn't affect type or binding. e.g. object initializers, probably match, etc. In an argument list it does affect binding (because of overload resolution), however, with named arguments it is not always significant.

[DavidArno]

@zippec,

It's hurting readability
How? Whats so confusing about it?

I refer you to my answer to a near identical question from a year ago:

---

"I'm aware it's optional; that's it's problem. When looking at a piece of code with a trailing comma, one must ask: is it there because the author likes to use them, because they forgot to refactor, or because something has been missed and thus it could be a bug. They create a serious hindrance to readability just to pander to developers who want their lives made fractionally easier when writing the code."

---

Frankly, I do not understand why would anyone oppose this improvement.

Because some of us see their use as indicative of sloppy programming practices that favour making code easy to write over easy to read.

[svick]

@alrz

In an argument list it does affect binding (because of overload resolution)

Do you have an example? I tried a trivial example of adding the trailing comma in current compiler and the error I got was:

error CS1525: Invalid expression term ')'

It seems the compiler never reached the overload resolution phase.

[orthoxerox]

I have to write a lot of SQL at work, and the lack of trailing commas is the bane of every SQL programmer (any probably of every programmer that has to write JSON, too). While I do not think the arguments are edited that often to really require support for trailing commas, I also do not oppose this idea.

[alrz]

@svick

I meant the number of arguments, for example you could add property initializers without changing the return type, so it's safe to allow it there. But if you add an argument it might change the selected method.

[HaloFour]

punctuation means something to scatter it about just to placate formatting or decades-old diff technology is silly we have refactoring tools which manage the proper placement of commas on reordering members or arguments

[jnm2]

I'm torn. On one hand trailing commas irritate me whenever I see them; on the other hand, it would be more consistent with the rest of the language.

Much as I instinctively dislike this, punctuation exists to serve the wishes of those who use it and no one would be forcing me to use it that way.

[alrz]

As it is a matter of code style, an analyzer could preserve, insert, remove or prohibit usages in the code, so I wouldn't mind even if it would be allowed everywhere in the language that we have a list-like construct.

[MgSam]

@HaloFour It has nothing to do with diff tools. Anyone who does a lot of editing of code with long method invocations with many named parameters runs into this. As @orthoxerox said, the problem is even more acute in SQL, and in my personal experience, some of those invocations of which I speak are calls to SQL stored procs or tables.

The fact that JS is adding this feature in 2017 means that a committee comprised heavily from the biggest tech companies ratified it. That's a pretty big hurdle to cross. It's not some poorly thought out artifact left over from the language's inception like some other JS features. I think that's a pretty good indication that the use case here is real and pervasive.

[HaloFour]

@MgSam

Anyone who does a lot of editing of code with long method invocations with many named parameters runs into this.

Indeed I have. Many, many, many times. Particularly with SQL and particularly with stored procs with dozens of parameters. Or tables with dozens to hundreds of fields. It's of the most minor of inconveniences and not one that I feel deserves special punctuation rules. Developers should probably be forced to manually fix their code more often after a bad copy&paste.

[MgSam]

It's definitely minor, and it only takes a second or two to correct. But correcting interrupts your train of thought. And when this happens a few times a day, everyday, the time adds up.