Covariance and contravariance — C# Language — Basics to Advanced

25. Covariance and contravariance

AdvancedMediumLesson 25 of 38

Variance answers: "if Dog is an Animal, can a Something<Dog> be used where a Something<Animal> is expected?"

Term	Direction	Keyword	Works when T is…	Example
Covariance	`Dog` → `Animal` (more derived to less)	`out T`	only returned (output)	`IEnumerable<out T>`, `Func<out TResult>`, `IReadOnlyList<out T>`
Contravariance	`Animal` → `Dog` (less derived to more)	`in T`	only accepted (input)	`Action<in T>`, `IComparer<in T>`, `Predicate<in T>`
Invariance	Exact type only	—	both in and out	`List<T>`, `IList<T>`, `Dictionary<K,V>`

csharp

class Animal { public string Name = ""; }
class Dog : Animal { }

IEnumerable<Dog> dogs = new List<Dog> { new() { Name = "Rex" } };
IEnumerable<Animal> animals = dogs;          // ✅ covariance: you only read Animals out of it

Action<Animal> feed = a => Console.WriteLine($"Feeding {a.Name}");
Action<Dog> feedDog = feed;                  // ✅ contravariance: something that handles any Animal can handle a Dog

Func<Dog> makeDog = () => new Dog();
Func<Animal> makeAnimal = makeDog;           // ✅ covariant return

// List<Animal> list = new List<Dog>();      // ❌ invariant: otherwise you could Add(new Cat()) into a list of dogs

// Your own variant interface
public interface IProducer<out T> { T Produce(); }          // T only in output positions
public interface IConsumer<in T> { void Consume(T item); }   // T only in input positions

Arrays are covariant too (object[] objs = new string[3];), but it's unsafe: objs[0] = 1; compiles and throws ArrayTypeMismatchException at runtime — one reason to prefer generic collections.

Interview tip: Remember "out = things come out (return values) → covariance; in = things go in (parameters) → contravariance". Variance works only for interfaces and delegates, and only with reference types (IEnumerable<int> is not an IEnumerable<object>).