Unsafe Code, Span<T> & High-Performance C#

using System;

class Program
{
    static void Main()
    {
        // A Span<T> is a "window" over memory you already have.
        // It copies NOTHING — it just remembers "start here, this many items".
        int[] numbers = { 10, 20, 30, 40, 50, 60, 70, 80 };

        // .AsSpan() makes a Span over the WHOLE array (zero allocation).
        Span<int> all = numbers.AsSpan();
        Console.WriteLine($"Length: {all.Length}");        // Length: 8

        // Slice(start, length) makes a small
...

using System;

class Program
{
    static void Main()
    {
        // 🎯 YOUR TURN — replace each ___ then press "Try it Yourself".
        int[] scores = { 5, 10, 15, 20, 25, 30 };

        // 1) Make a Span over the WHOLE array (no copy)
        Span<int> span = scores.___();      // 👉 AsSpan

        // 2) Slice the MIDDLE FOUR: start at index 1, take 4 items -> {10,15,20,25}
        Span<int> middle = span.Slice(1, ___); // 👉 the count: 4

        // 3) Add up the slice WITHOUT copying — 
...

using System;

class Program
{
    static void Main()
    {
        // 🎯 YOUR TURN — two no-allocation windows, no /unsafe needed.

        // PART A — a tiny scratch buffer on the STACK (no 'new', no GC).
        // stackalloc gives a Span<int> backed by stack memory.
        Span<int> buffer = ___ int[3];      // 👉 stackalloc
        buffer[0] = 1;
        buffer[1] = 2;
        buffer[2] = 3;
        Console.WriteLine($"buffer total = {buffer[0] + buffer[1] + buffer[2]}");

        // PART 
...

using System;
using System.Buffers;

class Program
{
    static void Main()
    {
        // Memory<T> is Span<T>'s heap-friendly cousin: it CAN be stored in
        // a field and used across 'await' (Span<T> cannot — see below).
        int[] data = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        Memory<int> memory = data.AsMemory();

        // Slice the same no-copy way:
        Memory<int> slice = memory.Slice(3, 4);            // {4,5,6,7}

        // To actually touch the elements, get a Span 
...

using System;

class Program
{
    // ⚠️ This whole method needs the /unsafe compiler flag
    // (<AllowUnsafeBlocks>true</AllowUnsafeBlocks> in the .csproj).
    // It's here to SEE pointers — the in-browser runner may refuse it.
    static unsafe void Main()
    {
        // & takes the address of a variable; int* is "pointer to int".
        int value = 42;
        int* ptr = &value;
        Console.WriteLine($"*ptr = {*ptr}");          // ✅ Expected output: *ptr = 42

        // 'fixed' PIN
...