Collection Literals – Possible to satisfy compile-time constant requirement?

[just-ero]

Would it be possible to allow collection literals to be passed, for example, as default parameters?

Something like this:

void M(int[] foo = [ ]) { }

today throws the compiler error Default parameter value for 'foo' must be a compile-time constant.

It would be fantastic to see this working one day.

[huoyaoyuan]

The limitation is from compiled assembly encoding, not the compile-time constant check. Currently, no collection type can be encoded in constants.

However for attributes we can encode array. Collection expressions may be permitted there.

[CyrusNajmabadi]

There is no way that that can be constant. Consider:

Int[] M(int[] foo = [ ]) {
   Foo[0] = 1;
   Return foo;

If this was constant, you'd now be mutating the constant value.

[jnm2]

Empty arrays are immutable, so there would have to be something inside the [].

[HaloFour]

Would kinda be nice to be able to have a default value of an empty array. C# could encode that as a custom attribute as it does with other pseudo-constants.

[TonyValenti]

@just-ero - for situations like that, you might consider using a functor approach or an args object with defaults on it.

[jnm2]

What are the motivating scenarios, things that get easier to do as a result?

[just-ero]

Optional array (or other collections) parameters like shown in the example in the original post.

[jnm2]

Having concrete scenarios helps get people interested.

[MitchRazga]

Does anyone know if collection expressions will be permitted within attributes in C#12?

I can't find any discussion within the LDM or working group notes but the analyzer IDE0300 ("Collection initialization can be simplified") suggests changing this:

[UnmanagedCallConv(CallConvs = new Type[] { typeof(CallConvCdecl) })]

to:

[UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]

which is currently invalid (CS0182).

[jnm2]

Support is coming soon, just not implemented yet. dotnet/roslyn#69133 is the issue to follow.