Kod niebezpieczny, wskaźniki i bufory o stałym rozmiarze

Czym jest kod unsafe?

// Must add to .csproj:
// <AllowUnsafeBlocks>true</AllowUnsafeBlocks>

// Unsafe blocks can appear inside methods:
unsafe
{
    int x = 42;
    int* ptr = &x;          // get address of x
    Console.WriteLine(*ptr); // dereference — prints 42
    *ptr = 100;
    Console.WriteLine(x);    // 100 — x was mutated via pointer
}

Typy wskaźnikowe w języku C#

unsafe
{
    int    i = 10;
    double d = 3.14;

    int*    ip = &i;
    double* dp = &d;

    // Dereference with *
    Console.WriteLine(*ip); // 10

    // Pointer arithmetic — move to next int in memory
    int[] arr = { 1, 2, 3 };
    fixed (int* p = arr)
    {
        Console.WriteLine(*(p + 0)); // 1
        Console.WriteLine(*(p + 1)); // 2
        Console.WriteLine(*(p + 2)); // 3
    }
}

Instrukcja fixed

byte[] buffer = new byte[256];

unsafe
{
    fixed (byte* pBuf = buffer)
    {
        // pBuf is valid only inside this block
        // GC won't relocate 'buffer' here
        for (int i = 0; i < buffer.Length; i++)
            pBuf[i] = (byte)i;
    }
    // After 'fixed', GC can move 'buffer' again
}

Console.WriteLine(buffer[5]); // 5

Arytmetyka wskaźników

unsafe
{
    int[] data = { 10, 20, 30, 40, 50 };
    fixed (int* start = data)
    {
        int* p = start;
        long sum = 0;
        for (int i = 0; i < data.Length; i++)
        {
            sum += *p;
            p++;    // advance by sizeof(int) = 4 bytes
        }
        Console.WriteLine(sum); // 150

        // Or with index syntax:
        Console.WriteLine(start[2]); // 30
    }
}

stackalloc: alokacja na stosie

// Stack-allocated buffer — zero heap allocation
Span<int> buffer = stackalloc int[128]; // safe Span wrapper
buffer.Fill(0);
buffer[0] = 42;
Console.WriteLine(buffer[0]); // 42

// Or raw pointer form (requires unsafe):
unsafe
{
    int* raw = stackalloc int[128];
    raw[0] = 99;
    Console.WriteLine(raw[0]); // 99
}
// Stack frame popped — memory gone

Bufory o stałym rozmiarze w strukturach

unsafe struct NetworkPacket
{
    public int  Length;
    public byte Command;
    public fixed byte Payload[256]; // 256 bytes inline in the struct
}

unsafe
{
    NetworkPacket pkt;
    pkt.Length  = 10;
    pkt.Command = 0x01;

    // Fill payload inline — no heap allocation
    for (int i = 0; i < 10; i++)
        pkt.Payload[i] = (byte)i;

    Console.WriteLine(pkt.Payload[5]); // 5
}
// sizeof(NetworkPacket) = 4 + 1 + 256 = ~261 bytes on stack

void* i rzutowanie

unsafe
{
    int value = 42;
    void* vp = &value;      // typeless pointer
    int*  ip = (int*)vp;    // cast back to int*
    Console.WriteLine(*ip); // 42

    // sizeof works on unmanaged types
    Console.WriteLine(sizeof(int));    // 4
    Console.WriteLine(sizeof(double)); // 8
    Console.WriteLine(sizeof(long));   // 8
}

Klasy MemoryMarshal i Unsafe

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

// Reinterpret a Span<byte> as Span<int> — no copy, no unsafe keyword
byte[] raw = new byte[16];
Span<int> ints = MemoryMarshal.Cast<byte, int>(raw);
ints[0] = 12345678;
Console.WriteLine(ints[0]); // 12345678

// Unsafe.As<,> — reinterpret reference type (advanced)
ref int first = ref MemoryMarshal.GetReference(ints);
Unsafe.Add(ref first, 1) = 99999;
Console.WriteLine(ints[1]); // 99999

Kiedy używać unsafe, a kiedy nie

// GOOD: narrow unsafe scope
public static unsafe int SumBytes(ReadOnlySpan<byte> data)
{
    int total = 0;
    fixed (byte* p = data)
    {
        byte* end = p + data.Length;
        for (byte* cur = p; cur < end; cur++)
            total += *cur;
    }
    return total;
}

// BETTER for modern code: use SIMD via Vector<T> or hardware intrinsics
// which stay in managed code but still achieve native-level perf

Praktyczny przykład: analizowanie protokołu binarnego

[StructLayout(LayoutKind.Sequential, Pack = 1)]
unsafe struct MessageHeader
{
    public uint  Magic;        // 4 bytes
    public ushort Version;    // 2 bytes
    public ushort PayloadLen; // 2 bytes
    public fixed byte Id[16]; // 16 bytes GUID inline
}

unsafe ReadOnlySpan<byte> SerializeHeader(MessageHeader hdr)
{
    // Cast struct directly to bytes — zero copy
    return new ReadOnlySpan<byte>(&hdr, sizeof(MessageHeader));
}

unsafe MessageHeader ParseHeader(ReadOnlySpan<byte> buf)
{
    fixed (byte* p = buf)
        return *(MessageHeader*)p; // reinterpret cast
}

Szybkie sprawdzenie

Podsumowanie: kod unsafe, wskaźniki i bufory fixed