Consider offering C# analog of Rust let repeated_array = [0; 256]

[richlander]

Collection expressions are nice but there doesn't seem to be a good way to say "give me a 256 element array". I've hit this issue many times. It's not so bad when using arrays. It's more jarring when the alternative is stackalloc.

Here's what's supported today.

byte[] numbersEasyMode = [1, 2, 3, 4, 5];
byte[] numbersClassicMode = new byte[5];
Span<byte> spanEasyMode = [1, 2, 3, 4, 5];
Span<byte> spanClassicMode = stackalloc byte[5];

It would be very nice to have symmetric syntax that covered the classic scenario, like Rust has.

[CyrusNajmabadi]

A core concern we had when looking at this is how trivial it is to get things wrong. for example byte[] b = [0; 255]; is very different from byte[] b = [0, 255];

Having a comma/semicolon have such a dramatic effect on meaning is something we did not like.

That said, we could roll this into the https://github.com/dotnet/csharplang/blob/main/proposals/collection-expression-arguments.md proposal.

While we only support collection expr args for apis using new SomeConstructibleType, or collection builders (ImmutableArray.Create), or interfaces, we could extend this out to arrays/spans, with something like: [with(Default: 0, Length: 256)].

[jnm2]

Yes, thanks!

[CyrusNajmabadi]

@jnm2 i really like [.. .Repeat(0, 256)]. How goes the work to spec out be able to provide factories for interfaces? The Enumerable case is def interesting. Especially being able to use .Repeat and .Range. :)

[jnm2]

Good to know you like it! We shall see, perhaps tonight.

[Richiban]

Does this also mean that something like

foreach (var i in .Range(0, 10))
{
...
}

might work?

[jnm2]

A target type for foreach could be considered together with a target type for ..!

[huoyaoyuan]

I think top-level [0; 256] is not so useful. new int[256], stackalloc int[256] and new CollectionType(capacity) are all easily accessible.

In the other hand, inserting a sequence into larger collection expression is more useful, for example [section1, section2, section3, section4, [0] * 4 /*space for signature*/].