5.cs

public static long ComputeSumMMF(string fileName, 
    Func<IntPtr, long, long, int, (long, int)> sumComputer)
{
    using var f = MemoryMappedFile.CreateFromFile(fileName, FileMode.Open);
    using var fAccessor = f.CreateViewAccessor();
    var fHandle = fAccessor.SafeMemoryMappedViewHandle;
    var (sum, _) = sumComputer(fHandle.DangerousGetHandle(), (long) fHandle.ByteLength, 0, 0);
    return sum;
}