Simplify File.WriteAllTextAsync

[stephentoub]

Avoid the unnecessary ArrayPool usage and extra layer of buffering. The code ends up being much simpler, and also a bit faster.

Method	Toolchain	Mean	Ratio	Allocated
WriteAllTextAsync	\main\CoreRun.exe	1.560 ms	1.00	22 KB
WriteAllTextAsync	\pr\CoreRun.exe	1.486 ms	0.97	19 KB

using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

[MemoryDiagnoser]
public class Program
{
    public static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private static string s_contents = string.Concat(Enumerable.Range(0, 100_000).Select(i => (char)('a' + (i % 26))));
    private static string s_path = Path.GetRandomFileName();
    private static CancellationTokenSource s_cts = new();

    [Benchmark]
    public Task WriteAllTextAsync() => File.WriteAllTextAsync(s_path, s_contents, s_cts.Token);
}

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

Issue Details

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Avoid the unnecessary ArrayPool usage and extra layer of buffering. The code ends up being much simpler, and also a bit faster.

Method	Toolchain	Mean	Ratio	Allocated
WriteAllTextAsync	\main\CoreRun.exe	1.560 ms	1.00	22 KB
WriteAllTextAsync	\pr\CoreRun.exe	1.486 ms	0.97	19 KB

using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

[MemoryDiagnoser]
public class Program
{
    public static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private static string s_contents = string.Concat(Enumerable.Range(0, 100_000).Select(i => (char)('a' + (i % 26))));
    private static string s_path = Path.GetRandomFileName();
    private static CancellationTokenSource s_cts = new();

    [Benchmark]
    public Task WriteAllTextAsync() => File.WriteAllTextAsync(s_path, s_contents, s_cts.Token);
}

Author:	stephentoub
Assignees:	-
Labels:	area-System.IO
Milestone:	-