[C#] Reduce allocations when using ArrayPool

[ovska]

Describe the enhancement requested

Quick single-use buffers are rented using helper methods that cause a (small) allocation on every invocation due to the delegate capturing references to the reader/writer and other related variables. This somewhat defeats the purpose of using pooled arrays, especially small ones for quick operations such as reading the message length.

Here's a reference implementation for achieving roughly the same convenience without any allocations (outside the ones the pool might make):

public readonly struct ArrayLease : IDisposable
{
    private readonly byte[] _array;
    private readonly ArrayPool<byte> _pool;

    public ArrayLease(byte[] array, ArrayPool<byte> pool)
    {
        _array = array;
        _pool = pool;
    }

    public void Dispose()
    {
        _pool.Return(_array);
    }
}

public static ArrayLease RentReturn(this ArrayPool<byte> pool, int length, out Memory<byte> buffer)
{
    byte[] array = pool.Rent(length);
    buffer = array.AsMemory(0, length);
    return new ArrayLease(array, pool);
}

Example usage:

- await Buffers.RentReturnAsync(4, async (buffer) =>
+ using (Buffers.RentReturn(4, out Memory<byte> buffer))
  {
      int footerLength;
      checked
      {
          footerLength = (int)(BaseStream.Position - offset);
      }

      BinaryPrimitives.WriteInt32LittleEndian(buffer.Span, footerLength);

      await BaseStream.WriteAsync(buffer, cancellationToken).ConfigureAwait(false);
- }).ConfigureAwait(false);
+ }

Component(s)

C#