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The Mechanic: Manual Memory & Pointers

🏗️ The Mechanic: Manual Memory & Pointers

Before we trust the Garbage Collector (GC) to manage our memory, we must understand how to manage it ourselves. In high-performance systems, “Managed” code can sometimes be a bottleneck.

1. The Stack vs. The Heap

In .NET, memory is divided into two primary zones:

  • The Stack: Fast, local, and automatically cleaned up when a method returns. (Value Types).
  • The Heap: Flexible, long-lived, but requires the GC to clean up. (Reference Types).

🛠️ Lab: Manual Stack Allocation

We can bypass the Heap entirely for small buffers using stackalloc.

public unsafe void ProcessData()
{
    // Allocating memory directly on the stack
    int* numbers = stackalloc int[5];

    for (int i = 0; i < 5; i++)
    {
        numbers[i] = i * 10;
        Console.WriteLine($"Value at pointer: {*(numbers + i)}");
    }
}

2. Pointers and unsafe code

To use pointers in C#, you must mark the code block as unsafe. This tells the compiler: “I am taking responsibility for memory safety.”

🧩 The Analogy: The Safety Harness

  • Managed Code: You are in a modern car with airbags and automatic braking.
  • Unsafe Code: You are on a motorcycle. You have more control and speed, but if you hit a wall (buffer overflow), there is no protection.
public unsafe void Swap(int* a, int* b)
{
    int temp = *a;
    *a = *b;
    *b = temp;
}

3. Fixed Memory (Pinning)

The GC likes to move objects around in the Heap to defragment it. If we have a pointer to an object, and the GC moves it, our pointer becomes “dangling” (points to garbage).

We use the fixed keyword to Pin an object in place.

byte[] data = new byte[100];

fixed (byte* p = data)
{
    // The GC will NOT move 'data' while we are in this block.
    p[0] = 0xFF;
}

🧪 Professor’s Challenge: The Manual Buffer

Task: Build a CLI tool that uses stackalloc to parse a CSV line without creating a single string object.

  1. Use ReadOnlySpan<char> to view the line.
  2. Use pointers to identify the comma positions.
  3. Print the values directly from memory.