Lesson 12 of 40 Core C# Intermediate ⏱ 35 min

Generics — Types, Methods & Constraints

Write reusable, type-safe code with generic classes, methods, constraints, variance, and common generic patterns.

Part 1: Generic Classes

public class Stack<T>
{
    private readonly List<T> _items = new();
    public void Push(T item) => _items.Add(item);
    public T Pop() { var t = _items[^1]; _items.RemoveAt(_items.Count-1); return t; }
    public int Count => _items.Count;
}

Part 2: Type Constraints

// where T : class — reference type only
// where T : struct — value type only
// where T : new() — has parameterless constructor
// where T : IComparable<T> — implements interface

public static T Max<T>(T a, T b) where T : IComparable<T>
=> a.CompareTo(b) >= 0 ? a : b;

Max(3, 7); // 7
Max("apple","mango"); // "mango"

Part 3: Covariance & Contravariance

// out T = covariant (IEnumerable<Dog> → IEnumerable<Animal>)
IEnumerable<Dog> dogs = GetDogs();
IEnumerable<Animal> animals = dogs; // ✅ Works!

// in T = contravariant (IComparer<Animal> → IComparer<Dog>)
IComparer<Animal> animalCmp = GetComparer();
IComparer<Dog> dogCmp = animalCmp; // ✅ Works!

Part 4: Generic Repository Pattern

public interface IRepository<T> where T : class
{
    Task<T?> GetByIdAsync(int id);
    Task<List<T>> GetAllAsync();
    Task<T> AddAsync(T entity);
    Task DeleteAsync(int id);
}

Generics — Types, Methods & Constraints

Part 1: Generic Classes

Part 2: Type Constraints

Part 3: Covariance & Contravariance

Part 4: Generic Repository Pattern

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