C# 12/13 `[InlineArray]` Fixed-Size Buffer Architecture

1Concept

`[InlineArray(N)]` creates a contiguous fixed-size struct buffer directly on the stack or inline inside an object, avoiding separate heap array allocations while maintaining safe indexing.

2Architecture Diagram

[InlineArray(4)] Struct Layout in RAM:
┌──────────────┬──────────────┬──────────────┬──────────────┐
│ Element [0]  │ Element [1]  │ Element [2]  │ Element [3]  │ (Contiguous 16 Bytes on Stack)
└──────────────┴──────────────┴──────────────┴──────────────┘

3Code Example

using System;
using System.Runtime.CompilerServices;

[InlineArray(4)]
public struct FixedBuffer4<T>
{
    private T _element0;
}

public class InlineArrayDemo
{
    public static void Main()
    {
        var buffer = new FixedBuffer4<int>();
        buffer[0] = 10;
        buffer[1] = 20;
        buffer[2] = 30;
        buffer[3] = 40;

        Console.WriteLine($"Inline Buffer Items: {buffer[0]}, {buffer[1]}, {buffer[2]}, {buffer[3]}");
        Console.WriteLine("Stored 100% contiguously on the thread stack without heap pointer dereferencing.");
    }
}

4Expected Output

Inline Buffer Items: 10, 20, 30, 40
Stored 100% contiguously on the thread stack without heap pointer dereferencing.

5Key Takeaways

• ✓`[InlineArray]` provides high-speed fixed buffers without `unsafe` pointers.
• ✓Enforces strict bounds checks preventing buffer overruns.
• ✓Used heavily throughout .NET 9 internals (e.g. `DefaultInterpolatedStringHandler`).