Add net9 File.WriteAllBytes over a span - #606

One member — File.WriteAllBytes(string, ReadOnlySpan<byte>).

Most of this family was already done

Diffing the whole byte-writing surface of File across the reference assemblies turned up eight members missing below net9, and seven of them are already polyfilled: AppendAllBytes in both the array and span forms, both AppendAllBytesAsync overloads, both WriteAllBytesAsync overloads, and ReadAllBytesAsync. WriteAllBytes(string, ReadOnlySpan<byte>) was the only real gap.

Delegating to the array overload, deliberately

The neighbouring polyfills in this file open their own FileStream (FileMode.Create, FileShare.None, and so on). This one does not, because it does not have to: File.WriteAllBytes(path, bytes.ToArray()) inherits the BCL's own file mode, sharing and validation rather than my guess at them.

Verified against net11 that the span overload really is indistinguishable from the array overload:

• identical content for empty, one-byte, three-byte and 100KB payloads
• the same truncation of a longer existing file, including down to zero length, with the file still present
• the same creation of a missing file
• the same exception for a null, empty or whitespace path, a missing directory, and a path that is a directory

So the delegation is faithful, and the cost is the array copy, which is //Note:d along with the fact that it is only reached with an explicit ReadOnlySpan — a byte[] argument binds to the BCL overload, as it does today.

A .NET Framework difference the tests had to accommodate

My first version pinned the empty-path failure as ArgumentException:path. That passed everywhere except net462, where File.WriteAllBytes("", bytes) throws ArgumentException with no paramName, while .NET names it path. That is a BCL difference between frameworks, and the polyfill correctly inherits whichever the running framework has — the equivalence assertion against the array overload passed on net462 all along; only my Core-specific hard-coded expectation failed.

The tests now assert full equivalence with the array overload on every framework, and pin concrete shapes only where they are stable everywhere (ArgumentNullException:path, DirectoryNotFoundException), checking just the exception type for the empty and whitespace cases. The comment records why.

Also fixed

The existing WriteAllBytesAsync(string, ReadOnlyMemory<byte>, CancellationToken) carried a //Link: pointing at appendallbytesasync. Corrected.

Verification

Solution clean in Release, Consume clean across all 22 TFMs, tests green on net11.0 (1745), net10.0 (1745), net9.0 (1745), net8.0 (1742), net462 (1691), plus PublicTests, EmbeddedTests, UnsafeTests, NoRefsTests and NoExtrasTests.

API count 1161 → 1162.