Declaring arrays

One block, many values

int[] scores = new int[5];              // 5 zeros
int[] primes = { 2, 3, 5, 7, 11 };      // infer from initializer
string[] names = new[] { "an", "binh" };

An array is a fixed-size block of elements of one type. new int[5] allocates 5 elements and fills value-type arrays with the type's default (0, false); element-type arrays (string[]) fill with null. Length is chosen at creation and never changes — need more room later, you allocate a new array (Module 10's List<T> exists precisely for that).

Indexing and bounds

primes[0]      // 2 — first
primes[^1]     // 11 — last (hat counts from the end)
primes[5]      // throws IndexOutOfRangeException — valid indices are 0..4

The bounds check is real and immediate. Off-by-one errors (<= in a loop, arr[arr.Length]) are the classic crash; Length is the size, the last valid index is Length - 1.

Reference semantics: the aliasing trap

int[] a = { 1, 2, 3 };
int[] b = a;          // b is ANOTHER NAME for the same array
b[0] = 99;
Console.WriteLine(a[0]);   // 99 — a changed too

Arrays are reference types. Assignment copies the reference, not the elements — two variables pointing at one block. This is the model for classes (Module 11) met early, where it can hurt: pass an array to a method and the method can mutate your data. To get an independent copy: a.Clone(), a.ToArray() (LINQ), or copy in a loop.