image from memoryStream to base64 with little allocations

imageConverter.cs

using System.Buffers;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using ConverterTest;

namespace MyBenchmarks
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var q = new Test();
            q.Array();
            
          
            q.Spans();
            
           
            q.ArraysRent();


            var summary = BenchmarkRunner.Run(typeof(Program).Assembly);
        }
    }
}


using System.Buffers;
using BenchmarkDotNet.Attributes;

namespace ConverterTest;

[MemoryDiagnoser(true)]
public class Test
{
    private readonly MemoryStream _stream;  

    public Test()
    {
        using var fileStream = new FileStream("F:\\nipi\\kiuss\\src\\Dt.Kpsirs.ClientGateway\\bin\\Debug\\net6.0\\files\\bd\\bd84b2df-eea9-46bf-9571-99a8ebd08f72__", FileMode.Open, FileAccess.Read);
        _stream = new MemoryStream();
        fileStream.CopyTo(_stream);
        _stream.Seek(0, SeekOrigin.Begin);
    }
    

    [Benchmark]
    public string Spans()
    {
        _stream.Position = 0;
        Span<byte> numbers = stackalloc byte[_stream.Capacity];
        _stream.Read(numbers);
        return Convert.ToBase64String(numbers);
    }

    ArrayPool<byte> shared = ArrayPool<byte>.Shared;
    [Benchmark]
    public string ArraysRent()
    {
        _stream.Position = 0;
        var rentedArray = shared.Rent(_stream.Capacity);
        _stream.Read(rentedArray);
        var result = Convert.ToBase64String(rentedArray);
        shared.Return(rentedArray, true);
        return result;
    }
    [Benchmark]
    public string Array()
    {
        _stream.Position = 0;
        var result = Convert.ToBase64String(_stream.ToArray());
        return result;
    }
}

I guess it's bad example for arrayRent, it needs to be tested with processing several different size files in a row.