What is covariance and contravariance?

Question

ICSM Computer · Answer

Covariance and contravariance are concepts in C# that deal with type compatibility in generic interfaces, delegates, and method parameters/returns, especially when dealing with inheritance hierarchies.

They allow flexibility in assigning types that are related by inheritance — while maintaining type safety.

Covariance (Output flexibility — `out`)

Allows a more derived type to be used than originally specified.
Applies to return types (output).
Declared using the out keyword (for generic type parameters).
Common in interfaces like IEnumerable<out T> or delegates.

Example:

IEnumerable<string> strings = new List<string>();
IEnumerable<object> objects = strings; // ✅ Covariant: string → object

public interface ICovariant<out T>
{
    T GetItem(); // Output only
}

Contravariance (Input flexibility — `in`)

Allows a more generic (base) type to be used than originally specified.
Applies to method parameters (input).
Declared using the in keyword (for generic type parameters).
Common in delegates like Action<in T> or interfaces like IComparer<in T>.

Example:

Action<object> actObj = o => Console.WriteLine(o);
Action<string> actStr = actObj; // ✅ Contravariant: object ← string

public interface IContravariant<in T>
{
    void SetItem(T item); // Input only
}

Summary Table

Concept	Keyword	Direction	Applies To	Example
Covariance	`out`	Output	Return types	`IEnumerable<out T>`
Contravariance	`in`	Input	Parameters	`Action<in T>`

Real-World Use Case

Covariance

You can assign IEnumerable<string> to IEnumerable<object> because strings are objects:

IEnumerable<string> names = new List<string>();
IEnumerable<object> objs = names; // ✅

Contravariance

A method that accepts object can be used where a method accepting string is expected:

Action<object> log = x => Console.WriteLine(x);
Action<string> logStr = log; // ✅

When to Use

Use covariance when your generic type is used only for return values.
Use contravariance when your generic type is used only for method parameters.

forum

What is covariance and contravariance?

Can you answer this question?

1 Answers

Covariance (Output flexibility — out)

Example:

Contravariance (Input flexibility — in)

Example:

Summary Table

Real-World Use Case

Covariance

Contravariance

When to Use

Covariance (Output flexibility — `out`)

Contravariance (Input flexibility — `in`)