diff --git a/csharp-book/book.toml b/csharp-book/book.toml
index c2a1a7d..2c581a4 100644
--- a/csharp-book/book.toml
+++ b/csharp-book/book.toml
@@ -15,6 +15,7 @@ additional-js = ["mermaid.min.js", "mermaid-init.js"]
 
 [preprocessor.mermaid]
 command = "mdbook-mermaid"
+optional = true
 
 [output.html.playground]
 editable = true
diff --git a/csharp-book/src/ch05-data-structures-and-collections.md b/csharp-book/src/ch05-data-structures-and-collections.md
index f98b0e7..b9fcf2c 100644
--- a/csharp-book/src/ch05-data-structures-and-collections.md
+++ b/csharp-book/src/ch05-data-structures-and-collections.md
@@ -283,6 +283,62 @@ fn print_string(s: &str) {
 }
 ```
 
+### Modern C#: Span\<T\> and Inline Arrays
+
+C# has evolved beyond traditional arrays. `Span<T>` provides type-safe, contiguous memory views that can live on the stack, while Inline Arrays (C# 12) offer fixed-size stack buffers.
+
+```csharp
+// C# Span<T> - view into contiguous memory
+Span<int> span = stackalloc int[] { 1, 2, 3, 4, 5 };
+span[0] = 10;
+
+ReadOnlySpan<char> text = "Hello".AsSpan();
+
+// Method accepting any contiguous memory view
+void ProcessSpan(ReadOnlySpan<int> data)
+{
+    for (int i = 0; i < data.Length; i++)
+        Console.WriteLine(data[i]);
+}
+
+// Inline Arrays (C# 12) - fixed-size stack buffer
+[InlineArray(5)]
+struct IntBuffer
+{
+    private int _element;
+}
+```
+
+```rust
+// Rust &[T] / &mut [T] - borrowed view into contiguous memory
+let mut array = [1, 2, 3, 4, 5];
+let slice: &mut [i32] = &mut array;
+slice[0] = 10;
+
+let slice: &[i32] = &array;
+let text: &str = "Hello";
+
+// Function accepting any sequential data
+fn process_slice(data: &[i32]) {
+    for (i, num) in data.iter().enumerate() {
+        println!("Index {}: {}", i, num);
+    }
+}
+
+// Fixed-size arrays (stack allocated)
+let buffer: [i32; 5] = [0; 5];
+```
+
+| C# | Rust |
+|----|------|
+| `Span<T>` (ref struct, stack-only) | `&mut [T]` / `&[T]` (borrowed slice) |
+| `ReadOnlySpan<T>` | `&[T]` (immutable slice) |
+| `ReadOnlySpan<char>` / `string.AsSpan()` | `&str` (string slice) |
+| `[InlineArray(N)]` struct (C# 12) | `[T; N]` (fixed-size array) |
+| `stackalloc T[]` with `Span<T>` | `let arr: [T; N] = ...` (local array) |
+
+> **Key insight:** Rust's `&[T]` combines the role of C#'s `ArraySegment<T>`, `Span<T>`, and `ReadOnlySpan<T>` — it's a fat pointer (pointer + length) that works with arrays, vectors, and subslices. C#'s Inline Arrays map naturally to Rust's `[T; N]` arrays, which are also stack-allocated by default.
+
 ***
 
 ## Structs vs Classes
diff --git a/rust-patterns-book/src/ch04-phantomdata-types-that-carry-no-data.md b/rust-patterns-book/src/ch04-phantomdata-types-that-carry-no-data.md
index 2bc6ac0..3f1ac92 100644
--- a/rust-patterns-book/src/ch04-phantomdata-types-that-carry-no-data.md
+++ b/rust-patterns-book/src/ch04-phantomdata-types-that-carry-no-data.md
@@ -46,6 +46,7 @@ struct Slice<'a, T> {
 Use `PhantomData` to prevent mixing values from different "sessions" or "contexts":
 
 ```rust
+use std::cell::RefCell;
 use std::marker::PhantomData;
 
 /// A handle that's valid only within a specific arena's lifetime
@@ -55,33 +56,35 @@ struct ArenaHandle<'arena> {
 }
 
 struct Arena {
-    data: Vec<String>,
+    data: RefCell<Vec<String>>,
 }
 
 impl Arena {
     fn new() -> Self {
-        Arena { data: Vec::new() }
+        Arena { data: RefCell::new(Vec::new()) }
     }
 
     /// Allocate a string and return a branded handle
-    fn alloc<'a>(&'a mut self, value: String) -> ArenaHandle<'a> {
-        let index = self.data.len();
-        self.data.push(value);
+    fn alloc(&self, value: String) -> ArenaHandle<'_> {
+        let mut data = self.data.borrow_mut();
+        let index = data.len();
+        data.push(value);
         ArenaHandle { index, _brand: PhantomData }
     }
 
     /// Look up by handle — only accepts handles from THIS arena
-    fn get<'a>(&'a self, handle: ArenaHandle<'a>) -> &'a str {
-        &self.data[handle.index]
+    fn get<'a>(&'a self, handle: ArenaHandle<'a>) -> String {
+        let data = self.data.borrow();
+        data[handle.index].clone()
     }
 }
 
 fn main() {
-    let mut arena1 = Arena::new();
+    let arena1 = Arena::new();
     let handle1 = arena1.alloc("hello".to_string());
 
     // Can't use handle1 with a different arena — lifetimes won't match
-    // let mut arena2 = Arena::new();
+    // let arena2 = Arena::new();
     // arena2.get(handle1); // ❌ Lifetime mismatch
 
     println!("{}", arena1.get(handle1)); // ✅