How is args is { Length: > 0 } better than args.Length > 0

[maxkoshevoi]

I came across this code, and am a bit confused about why pattern matching is used here.
Isn't args.Length > 0 would be better (faster)?

runtime/src/libraries/Microsoft.Extensions.Hosting/src/HostingHostBuilderExtensions.cs

Line 194 in a14d27a

if (args is { Length: > 0 })

runtime/src/libraries/Microsoft.Extensions.Hosting/src/HostingHostBuilderExtensions.cs

Line 219 in a14d27a

if (args is { Length: > 0 })

[stephentoub]

There's a functional difference.

if (args is { Length: > 0 }) 

is the same as:

if (args is not null && args.Length > 0)

whereas

if (args.Length > 0)

will null ref if args is null. If args can't be null, if (args.Length > 0) is a tad better. If it can be null, if (args is { Length: > 0 }) is clearly better.

(But with the lines you cited, there's no way that null check will have a measurable impact on performance.)

[stephentoub]

Btw, do how know whether values in args could be null?

From an annotations perspective, it'd be string?[] args to indicate it may contain nulls and string[] args, but that of course doesn't guarantee it doesn't (e.g. string[] arr = new string[10] is an array filled with nulls even though it's annotated as containing non-null strings). To know whether it actually contains nulls at run-time, you'd need to iterate through the array.

[maxkoshevoi]

it'd be string?[] args to indicate it may contain nulls and string[] args

Sorry, didn't quite get you. So string?[] args or string[] args? Interesting point string[] arr = new string[10] being filled with nulls, and compiler not complaining about it, haven't thought about that.

[stephentoub]

string?[] args is a non-nullable array that might contain nulls.

string[]? args is a nullable array that shouldn't contain nulls.

string[] args is a non-nullable array that shouldn't contain nulls.

[maxkoshevoi]

Yeah, I understand that. I was asking you about that specific case (IHostBuilder.ConfigureDefaults).

runtime/src/libraries/Microsoft.Extensions.Hosting/src/HostingHostBuilderExtensions.cs

Line 188 in a14d27a

public static IHostBuilder ConfigureDefaults(this IHostBuilder builder, string[] args)

If you don't know off the top of your head, that's fine. Sorry for misunderstanding

[stephentoub]

I was asking you about that specific case (IHostBuilder.ConfigureDefaults)

I would assume string[]? args. I see callers passing in null, and otherwise the args come from Main, which shouldn't be getting null strings in the array.