ARRAY #####1##### 4050 ms

void Main()
{
    var a = new int[100000];
    for (int i = 0; i < a.Length; i++)
    {
        a[i] = (new Random()).Next(1, 100);
    }
    int repeats = a.Length - 1;
    var sw = Stopwatch.StartNew();
    do
    {
        a[repeats] = Sum(in a);
    }
    while (--repeats != 0);
    sw.Stop();
    Console.WriteLine(sw.ElapsedMilliseconds);
}

int Sum(in int[] arr)
{
    int sum = 0;
    for (int i = 0; i < arr.Length; i++)
    {
        sum += arr[i];
    }
    return sum;
}


ARRAY #####2##### 6209 ms

static void Main()
{
    var a = new int[100000];
    for (int i = 0; i < a.Length; i++)
    {
        a[i] = (new Random()).Next(1, 100);
    }
    int repeats = a.Length - 1;
    var sw = Stopwatch.StartNew();
    do
    {
        a[repeats] = Sum(in a);
    }
    while (--repeats != 0);
    sw.Stop();
    Console.WriteLine(sw.ElapsedMilliseconds);
}

static int Sum(in int[] arr)
{
    int sum = 0;
    for (int i = 0; i < arr.Length; i++)
    {
        sum += arr[i];
    }
    return sum;
}

####POINTERS##### 3990

static void Main()
{
    unsafe
    {
        int length = 100000;
        int* s = stackalloc int[length];
        int* sp = &s[length - 1];

        for (int i = 0; i < length; i++)
        {
            s[i] = (new Random()).Next(1, 100);
        }
        int repeats = length - 1;
        var sw = Stopwatch.StartNew();
        do
        {
            *sp = Sum(&s[0], length);
            sp--;
        }
        while (--repeats != 0);
        sw.Stop();
        Console.WriteLine(sw.ElapsedMilliseconds);
    }
}
static unsafe int Sum(int* arr, int length)
{
    int sum = 0;
    for (int i = 0; i < length; i++)
    {
        sum += *arr;
        arr++;
    }
    return sum;
}

#####SPAN##### 6300
static void Main()
{
    Span<int> s = stackalloc int[100000];
    for (int i = 0; i < s.Length; i++)
    {
        s[i] = (new Random()).Next(1, 100);
    }
    int repeats = s.Length - 1;
    var sw = Stopwatch.StartNew();
    do
    {
        s[repeats] = Sum(s);
    }
    while (--repeats != 0);
    sw.Stop();
    Console.WriteLine(sw.ElapsedMilliseconds);
}

static int Sum(Span<int> arr)
{
    int sum = 0;
    for (int i = 0; i < arr.Length; i++)
    {
        sum += arr[i];
    }
    return sum;
}