Use string.Create in BitConverter.ToString(byte[])

[justinvp]

Avoids unnecessary allocations and copying, and reduces the amount of unsafe code.

Benchmark:

using System;
using System.Diagnostics;
using System.Text;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Attributes.Jobs;
using BenchmarkDotNet.Running;

[MemoryDiagnoser]
[InProcess]
public class Program
{
    public static void Main() => BenchmarkRunner.Run<Program>();

    const string SmallData = "Lorem ipsum dolor sit amet";
    const string LargeData =
        "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do " +
        "eiusmod tempor incididunt ut labore et dolore magna aliqua. " +
        "Ipsum suspendisse ultrices gravida dictum fusce.";
    public static readonly byte[] s_smallData = Encoding.UTF8.GetBytes(SmallData);
    public static readonly byte[] s_largeData = Encoding.UTF8.GetBytes(LargeData);

    [Benchmark]
    public string BeforeSmall() => OldConverter.ToString(s_smallData);

    [Benchmark]
    public string AfterSmall() => NewConverter.ToString(s_smallData);

    [Benchmark]
    public string BeforeLarge() => OldConverter.ToString(s_largeData);

    [Benchmark]
    public string AfterLarge() => NewConverter.ToString(s_largeData);
}

Results:

BenchmarkDotNet=v0.10.10, OS=Windows 10 Redstone 2 [1703, Creators Update] (10.0.15063.726)
Processor=Intel Core i7-6700K CPU 4.00GHz (Skylake), ProcessorCount=2
Frequency=3914037 Hz, Resolution=255.4907 ns, Timer=TSC
.NET Core SDK=2.2.0-preview1-007558
  [Host] : .NET Core 2.1.0-preview1-25915-01 (Framework 4.6.25915.02), 64bit RyuJIT

Job=InProcess  Toolchain=InProcessToolchain

      Method |      Mean |     Error |    StdDev |  Gen 0 | Allocated |
------------ |----------:|----------:|----------:|-------:|----------:|
 BeforeSmall |  80.10 ns | 0.4675 ns | 0.4373 ns | 0.0437 |     184 B |
  AfterSmall |  77.63 ns | 0.2555 ns | 0.1995 ns | 0.0437 |     184 B |
 BeforeLarge | 464.79 ns | 1.6609 ns | 1.3870 ns | 0.5031 |    2112 B |
  AfterLarge | 457.05 ns | 2.2694 ns | 2.1228 ns | 0.2513 |    1056 B |

[stephentoub]

Nice. Thanks.