[asylkhan-azat]

Remove tmpByteBuffer and tmpStream from StreamReader.ReadAsyncInternal and ReadBufferAsync and use the readonly fields directly.

I've also run benchmarks with both runtime async enabled & disabled.

Details

[MemoryDiagnoser]
public class StreamReaderBenchmarks
{
    private string _path = null!;
    private readonly char[] _buffer = new char[4096];

    [GlobalSetup]
    public void Setup()
    {
        _path = Path.GetTempFileName();
        string line = new string('x', 127) + "\n";
        using var writer = new StreamWriter(_path, false, new UTF8Encoding(false));
        for (int i = 0; i < 8192; i++)
            writer.Write(line);
    }

    [GlobalCleanup]
    public void Cleanup() => File.Delete(_path);

    [Benchmark]
    public async Task<int> ReadAsync()
    {
        using var stream = new FileStream(_path, FileMode.Open, FileAccess.Read, FileShare.Read,
            4096, FileOptions.Asynchronous | FileOptions.SequentialScan);
        using var reader = new StreamReader(stream);
        int total = 0, count;
        while ((count = await reader.ReadAsync(_buffer.AsMemory())) != 0)
            total += count;
        return total;
    }

    [Benchmark]
    public async Task<int> ReadLineAsync()
    {
        using var stream = new FileStream(_path, FileMode.Open, FileAccess.Read, FileShare.Read,
            4096, FileOptions.Asynchronous | FileOptions.SequentialScan);
        using var reader = new StreamReader(stream);
        int total = 0;
        while (await reader.ReadLineAsync() is string line)
            total += line.Length;
        return total;
    }
}

Benchamrk results:

Details

BenchmarkDotNet v0.15.8, Windows 11 (10.0.26300.9818)
AMD Ryzen 5 7500F 3.70GHz, 1 CPU, 12 logical and 6 physical cores
.NET SDK 12.0.100-alpha.1.26480.102
  [Host] : .NET 12.0.0 (12.0.0-dev, 42.42.42.42424), X64 RyuJIT x86-64-v4

Job=ShortRun  Toolchain=InProcessEmitToolchain  IterationCount=15  
IterationTime=500ms  LaunchCount=1  WarmupCount=5  

Runtime async, before change

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
ReadAsync	2.775 ms	0.0184 ms	0.0163 ms	10.4167	-	180.99 KB
ReadLineAsync	3.129 ms	0.0217 ms	0.0203 ms	175.0000	6.2500	2928.83 KB

Runtime async, after change

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
ReadAsync	2.782 ms	0.0252 ms	0.0236 ms	5.6818	-	172.94 KB
ReadLineAsync	3.126 ms	0.0511 ms	0.0478 ms	176.1364	5.6818	2924.83 KB

Roslyn, before change

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
ReadAsync	2.994 ms	0.0608 ms	0.0569 ms	5.2083	-	88.52 KB
ReadLineAsync	3.370 ms	0.1088 ms	0.1018 ms	175.0000	6.2500	2878.56 KB

Roslyn, after change

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
ReadAsync	2.956 ms	0.0280 ms	0.0262 ms	-	-	80.52 KB
ReadLineAsync	3.322 ms	0.1617 ms	0.1513 ms	175.0000	6.2500	2874.54 KB

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[asylkhan-azat]

Btw, I've no idea why the runtime async is so much more expensive compared to roslyn

[unsafePtr]

This saved state was used for hoisting, so not sure if it's actually a good change

[asylkhan-azat]

This saved state was used for hoisting, so not sure if it's actually a good change

I guess this hoisting was needed because the object inheirts the MarshalByRefObject where the field accesses where a problem on older versions of NET Framework. I'm not sure if currently it earns it's keep

[jkotas]

Thanks