Skip to main content
📜 WAYPOINT LESSON

Generic collections are the payoff

⭐ beginner⏳ 11 min read📍 Lesson 48 of 85

`List<T>`, `Dictionary<TKey, TValue>`, `Queue<T>`, `Stack<T>` — the BCL is built from the same tool you just learned.

Reading the signatures

public class List<T> { ... }
public class Dictionary<TKey, TValue> { ... }

The collections you've used since module 8 are generic classes. Their type parameters are why list[0] returns T — not object you must cast — and why putting a string into a List<int> is a compile error, not a runtime surprise.

Why not object?

Pre-generics collections stored object (the base of everything):

// the old disease, still legal:
var list = new ArrayList();
list.Add(42);
int x = (int)list[0];          // cast needed, checked at RUNTIME
list.Add("oops");              // compiles fine — boom later

Value types stored in object also box (heap-allocate) on every add. Generics eliminate the casts, move errors to compile time, and skip the boxing. That trio — no casts, compile-time safety, no boxing — is the entire point.

⚡ Now practice

Ready to Code
Generics workoutSwaps, stacks, pair stores, and a count-if — type parameters under discriminating tests.
4 challenges · · ~40 min