Allow comma separated var declarations

[drewnoakes]

JavaScript/TypeScript allow separating var declarations with a comma, where each declaration's type may differ. It makes for easily readable phrases. For example:

var radius = 3,
    point = GetPoint(),
    x = radius*2*point.X + radius,
    y = radius*2*point.Y + radius;

Right now comma separated declarations only seem to work when the type is specified explicitly, in which case it is restricted to being identical amongst all declared variables.

This is just a syntax thing. In my opinion, the above code is easier to read. The compiler exhibits no preference.

This request applies equally to the proposed readonly locals (#155), whatever keyword is decided upon (val/let/readonly var/etc).

[dsaf]

var in C# is completely different from var in JavaScript/TypeScript. C# has JavaScript's var as dynamic so you can do what JavaScript does already today:

dynamic a = 3,
        b = "name",
        c = DateTime.Now;

What would IDE even show when you hover over var?

[sharwell]

It makes for easily readable phrases.

This is an interesting proposal. I actually believe the opposite for this (i.e. that it makes for especially unreadable phrases). However, if the syntax were allowed then StyleCopAnalyzers could be updated to identify cases where the syntax was used and automatically convert it to another form. The overall advantage would be reducing the amount of typing required to input certain programs.

What would IDE even show when you hover over var?

This is certainly something to think about, but not really a reason to eliminate the feature. Similar problems arose for nameof (especially regarding CodeLens and rename refactorings), but they worked themselves out over time.

[dsaf]

There is one short step from that to this:

public class Sample
{
    public var Name { get; } = "Abcde";
    public var Properties { get; } = new Dictionary<string, object>();
}

Is it the right direction for C#?

[sharwell]

There is one short step from that to this:

Right now you can use var for local variables, but not for fields constants. It seems like using it for properties would be quite a long step away, and also an orthogonal request.

[dsaf]

By the way there is a reason why it's not already implemented: http://stackoverflow.com/a/4950600/486561

[HaloFour]

And also note that this was a point of contention with C++ auto. They did implement multiple declarations but all of the variables must always deduce to the same type. The type inference won't even consider common base classes:

// given Bar : Foo and Baz : Foo
// ERROR C3538 in a declarator-list 'auto' must always deduce to the same type
// WARNING 'auto' type is "Bar" for this entry, but was previously implied to be "Foo"
// WARNING 'auto' type is "Baz" for this entry, but was previously implied to be "Foo"
auto foo = Foo(), bar = Bar(), baz = Baz();

I'd have to agree with Eric Lippert. Allowing the syntax would produce confusion as a decent percentage of people would expect whatever was opposite to the implemented behavior.

[jveselka]

You can implements the most strict option and no confusion can arise. "All variables in declaration must be initialized to the same type with no inferrence allowed." It still has value over prohibiting comma separated var declaration altogether.

var a = 1, b = "2"; //not OK
var c = 3, d = 4.0; //not OK
var e = 5, f = 6; //OK
var g = 7, h = 2147483648; //not OK (int and uint)
var i = 9u, j = 2147483648; //OK

Meaning of code is super-obvious and all attempts for misuse fail at decalration.

[CyrusNajmabadi]

I have a PR out for this here: #5048

[gafter]

We've decided not to do this. The syntactic savings are tiny.