Champion: Switch expression as a statement expression

[gafter]

I propose that we support a switch expression as a statement expression when every arm's expression is also a statement expression. No common type among the arms is required when used as a statement expression.

void M(bool c, ref int x, ref string s)
{
    c switch { true => x = 1, false => s = null };
}

Meeting notes

https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-08-31.md#switch-expression-as-a-statement

[DavidArno]

Just to check, since I get confused between statement expressions and expression statements, will the following syntax by valid with this proposal:

void M(bool c, ref int x, ref string s) => c switch { true => x = 1, false => s = null };

[Mteheran]

Is the operator for default case still the same for this scenario or this new feature??

As I understand underscore is the current operator

void M(bool c, ref int x, ref string s) { c switch { true => x = 1, false => s = null, _ => x=0 }; }

[dsaf]

Together with exhaustiveness checks this will render existing switch statement pretty much legacy?

[CyrusNajmabadi]

@dsaf how so? You still can't have things like blocks/multi-statements in an expression-form switch.

[dsaf]

@CyrusNajmabadi well maybe initially but those could be added going forward.

[alrz]

Together with exhaustiveness checks this will render existing switch statement pretty much legacy?

Non-exhaustive switches can't be written via switch expression,

void M(string s) {
    switch (o) {
        case 1: s = "A"; break; 
        case 2: s = "B"; break;
    }
}

Unless this is allowed

void M(string s) {
    o switch { 1 => s = "A", 2 => s = "B" };
}

(You still need to repeat the assignment.)

[CyrusNajmabadi]

@CyrusNajmabadi well maybe initially but those could be added going forward.

In that case, they do not "render existing switch statement pretty much legacy". :)

[Thaina]

What happen to

var a = c switch { true => x = 1, false => s = null };

?

[DavidArno]

@Thaina,

It seems sensible that it'll give you a error CS0815: Cannot assign void to an implicitly-typed variable

[Thaina]

@DavidArno We actually could write this

int x = 0;
var a = x = 1; // a is int

And in the next C# I think we could possible to

var a = condition ? 1 : null; // a become Nullable<int>

So actually I expect that it might be possible to

var a = c switch { true => x = 1, false => s = null };
// a become Nullable<int> or object or (int | string)
// then assign both x and a to 1 if true else assign both s and a to null

So I just ask to make sure

[yaakov-h]

If s is a string, then you get error CS8506: No best type was found for the switch expression.

If s is a Nullable<int>/int?, that already compiles, and I see no reason why that would change.

[Thaina]

@yaakov-h It possible that it's behaviour could be CS0815 as @DavidArno speculate. Or maybe other. That's why I ask to make sure what it really would be

As for me I hope that we should support union type and could let this syntax return union type. Or just plain object

[DavidArno]

@Thaina,

The value of, c switch { true => x = 1, false => s = null };, isn't ?int though; it's void. Just as I can write,

void Foo() {}
void Bar() => Foo();
void Baz()
{
    var x = Foo(); // results in error CS0815: Cannot assign void to an implicitly-typed variable
}

So the same should be true of a statement expression switch expression,

void Bar(bool c, ref int x, ref string s) 
    => c switch { true => x = 1, false => s = null }; // all good
void Baz(bool c, ref int x, ref string s)
{
    var a = switch { true => x = 1, false => s = null }; 
    // results in error CS0815: Cannot assign void to an implicitly-typed variable
}